Lesson 1: The basics of C++

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Lesson 1: The basics of C++
This tutorial is for everyone; if you've never programmed before, or if you have extensive experience programming in other languages and want to expand into C++. It is for everyone who wants the feeling of accomplishment from a working program.

C++ is a programming language of many different dialects, just as each spoken has many different dialects. In C++, dialects are not because the speakers live in the North or South; it is because there are several different compilers. There are four major compilers: Borland C++, Microsoft Visual C++, Watcom C/386, and DJGPP. You can download DJGPP or you may already have another compiler. 

Each of these compilers is slightly different. Each one will support the ANSI/ISO standard C++ functions, but each compiler will also have nonstandard functions (these functions are similar to nonstandard words spoken in different parts of the country. For example, in New Orleans the median is called the neutral ground). Sometimes the use of nonstandard functions will cause problems when you attempt to compile source code (the actual C++ written by a programmer) in a different compiler. These tutorials should not suffer from this problem.
If you don't have a compiler, I strongly suggest you get a compiler. A simple compiler is sufficient for these tutorials, but you should get one in order to follow them. 

C++ is a different breed of programming language. For DOS-based C++ compilers, there are only a few keywords; keywords alone are insufficient for input or output functions. Almost all usesful functions are in library files that can be accessed by including header files. Let's look at a real program:


#include <iostream.h>
int main()
{
  cout<<"HEY, you, I'm alive!  Oh, and Hello World!";
  return 0;    
}
Let's look at the elements of the program. The #include is a preprocessor directive that tells the compiler to put code from the header file iostream.h into our program. By including header files, you an gain access to many different functions. For example, the cout function requires iostream.h.


The next imporant line is int main(). This line tells the compiler that there is a function named main, and that the function returns an integer, hence int. The braces ({ and }) signal the beginning and end of functions and other code blocks. If you have programmed in Pascal, you will know them as BEGIN and END. 

The next line of the program may seem strange. If you have programmed in another language, you might expect that print would be the function used to display text. In C++, however, the cout function is used to display text. It uses the <;<; symbols, known as insertion operators. The quotes tell the compiler that you want to output the literal string as-is. The semicolon is added onto the end of all function calls in C++; the semicolon later shows up when you declare variables.

The penultimate line of code orders main to return 0. When one returns a value from main, it is passed on to the operating system. As a note, declaring main as though it had no return type - void main() - usually works; it is considered bad practice, however.

The final brace closes off the function. You should try running this program in a compiler. You can cut and paste the code into a file, save it as a .cpp file, and then open the file from within a compiler. If you are using a command-line compiler, such as Borland C++ 5.5, you should read the compiler instructions for information on how to compile.

Comments are often useful when programming. When you tell the compiler a section of text is a comment, the compiler will ignore it when running the code. To create a comment use either //, which tells the compiler that the rest of the line is a comment, or /* and then */ to block off everything between as a comment. Certain compilers will change the color of a commented area, but some will not. Be certain not to accidentally comment out code (that is, to tell the compiler part of your code is a comment) you need for the program. When you are learning to program, it is useful to be able to comment out sections of code in order to see how the output is affected.

So far you should be able to write a simple program to display information typed in by you, the programmer. It is also possible for your program to accept input. the function you use is known as cin, and is followed by the insertion operator >>.

Of course, before you try to receive input, you must have a place to store that input. In programming, input and data are stored in variables. There are several different types of variables; when you tell the compiler you are declaring a variable, you must include the data type along with the name of the variable. Several basic types include char, int, and float.

A variable of type char stores a single character; variables of type int store integers (numbers without decimal places), and variables of type float store numbers with decimal places. Each of these variable types - char, int, and float - is each a keyword that you use when you declare a variable. To declare a variable you use the syntax type <name>. It is permissible to declare multiple variables of the same type on the same line; each one should be separated by a comma. The declaration of a variable or set of variables should be followed by a semicolon (Note that this is the same procedure used when you call a function). If you attempt to use an undefined variable, your program will not run, and you will receive an error message informing you that you have made a mistake.

Here are some variable declaration examples:

int x;
int a, b, c, d;
char letter;
float the_float;

While you can have multiple variables of the same type, you cannot have multiple variables with the same name. Moreover, you cannot have variables and functions with the same name.


#include <iostream.h>
int main()
{
  int thisisanumber;
  cout<<"Please enter a number:";
  cin>>thisisanumber;
  cout<<"You entered: "<<thisisanumber;
  return 0;
}
Let's break apart this program and examine it line by line. The keyword int declares thisisanumber to be an integer. The function cin>> reads a value into thisisanumber; the user must press enter before the number is read by the program. Keep in mind that the variable was declared an integer; if the user attempts to type in a decimal number, it will be truncated (that is, the decimal component of the number will be ignored). Try typing in a sequence of characters or a decimal number when you run the example program; the response will vary from input to input, but in no case is it particularly pretty. Notice that when printing out a variable quotation marks are not used. Were there quotation marks, the output would be "You Entered: thisisanumber." The lack of quotation marks informs the compiler that there is a variable, and therefore that the program should check the value of the variable in order to replace the variable name with the variable when executing the output function. Do not be confused by the inclusion of two separate insertion operators on one line. Including multiple insertion operators on one line is acceptable as long as each insertion operator outputs a different piece of information; you must separate string literals (strings enclosed in quotation marks) and variables by giving each its own insertion operators (<<). Trying to put two variables together with only one <;<; will give you an error message; do not try it. Do not forget to end functions and declarations with a semicolon. If you forget the semicolon, the compiler will give you an error message when you attempt to compile the program. 

Variables are uninteresting without the ability to modify them. Several operators used with variables include the following: *, -, +, /, =, ==, >, <. The * multiplies, the - subtracts, and the + adds. It is of course important to realize that to modify the value of a variable inside the program it is rather important to use the equal sign. In some languages, the equal sign compares the value of the left and right values; but in C++ == is used for that task. The equal sign is still extremely useful. It sets the left input to the equal sign, which must be one, and only one, variable, equal to the value on the right side of the equal sign. The operators that perform mathematical functions should be used on the right side of an equal sign in order to assign the result to a variable on the left side. 

Here are a few examples:

a=4*6;  //(Note use of comments and of semicolon) a is 24
a=a+5;  // a equals the original value of a with five additional units
a==5    //Does NOT assign five to a.  Rather, it checks to see if a equals 5.

The other form of equal, ==, is not a way to assign a value to a variable. Rather, it checks to see if the variables are equal. It is useful in other areas of C++; for example, you will often use == in such constructions as if statements and loops. 

You can probably guess how < and > function. They are greater than and less than operators . 
For example: 
a<5       //Checks to see if a is less than five
a>5   //Checks to see if a is greater than five
a==5      //Checks to see if a equals five, for good measure

Lesson 2: If statements
The ability to control the flow of your program, whether or not your program will run a section of code based on specific tests, is valuable to the programmar. The if statement allows you to control if a program enters a section of code or not based on whether a given condition is true or false. One of the important functions of the if statement is that it allows the program to select an action based upon the user's input. For example, by using an if statement to check a user entered password, your program can decide whether a user is allowed access to the program .

Without a conditional statement such as the if statement, programs would run almost the exact same way every time. If statements allow the flow of the program to be changed, and so they allow algorithms and more interesting code.

Before discussing the actual structure of the if statement, let us examine the meaning of TRUE and FALSE in computer terminology. A true statement is one that evaluates to a nonzero number. A false statement evaluates to zero. When you perform comparison checks, the operator will return a nonzero number if the comparison is true, or zero if the comparison is false. For example, the check 0==2 evaluates to 0. The check 2==2 evaluates to a nonzero number, although we cannot be certain of its value. If this confuses you, try to use a cout statement to output the result of those various comparisons (for example cout<<2==1;)

When programming, the aim of the program will often require the checking of one value stored by a variable against another value to determine whether one is larger, smaller, or equal to the other.

There are a number of operators that allow these checks.

Here are the relational operators, as they are known, along with examples:

>     greater than              5>4 is TRUE
<     less than                 4<5 is TRUE
>=    greater than or equal     4>=4 is TRUE
<=    less than or equal        3<=4 is TRUE

It is highly probable that you have seen these before, probably with slightly different symbols. They should not present any hindrance to understanding.

Now that you understand TRUE and FALSE in computer terminology as well as the comparison operators, let us look at the actual structure of if statements.

The structure of an if statement is as follows: 
if (TRUE)
  Do whatever follows on the next line.

To have more than one statement execute after an if statement that evaluates to true, use brackets. 

For example: 
if (TRUE)
{
  Do everything between the brackets.
}
There is also the else statement. The code after it (whether a single line or code between brackets) is executed if the IF statement is FALSE.

It can look like this:

if(FALSE)
{ 
  Not executed if its false
}
else 
{
  do all of this
}

One use for else is if there are two conditional statements that may both evaluate to true, yet you wish only one of the two to have the code block following it to be executed. You can use an else if after the if statement; that way, if the first statement is true, the else if will be ignored, but if the if statement is false, it will then check the condition for the else if statement. If the if statement was true the else statement will not be checked. It is possible to use numerous else if statements.

Let's look at a simple program for you to try out on your own.

#include <iostream.h>   
int main()    //Most important part of the program!
{
  int age;    //Need a variable...
  cout<<"Please input your age: "; //Asks for age
  cin>>age;    //The input is put in age
  if(age<100)    //If the age is less than 100
  {
     cout<<"You are pretty young!";     //Just to show it works
  }
  else if(age==100)  //I use else just to show an example 
  {
     cout<<"You are old";  //Just to show you it works...
  }
  else
  {
    cout<<"You are really old"; //Executed if no other statement is executed
  }
  return 0;
}

Boolean operators allow you to create more complex conditional statements. For example, if you wish to check if a variable is both greater than five and less than ten, you could use the boolean AND to ensure both var>5 and var<10 are true. In the following discussion of boolean operators, I will capitalize the boolean operators in order to distinguish them from normal english. The actual C++ operators of equivalent function will be described further into the tutorial - the C++ symbols are not: OR, AND, NOT, although they are of equivalent function.

When using if statements, you will often wish to check multiple different conditions. You must understand the Boolean operators OR, NOT, and AND.


The boolean operators function in a similar way to the comparison operators: each returns 0 if evaluates to FALSE or a nonzero number if it evaluates to TRUE. 

NOT: The NOT operator accepts one input; if that input is TRUE, it returns FALSE, and if that input is FALSE, it returns TRUE. For example, NOT (1) evalutes to 0, and NOT (0) evalutes to some nonzero. NOT (any number but zero) evaluates to 0. In C and C++ NOT is written as !. NOT is evaluated prior to both AND and OR. 

AND: This is another important command; AND returns TRUE if both inputs are TRUE (if 'this' AND 'this' are true). (1) AND (0) would evaluate to zero because one of the inputs is false (both must be TRUE for it to evaluate to TRUE). (1) AND (1) evaluates to some nonzero number. (ANY NUMBER BUT ZERO) AND (0) evaluates to 0. (ANY REAL NUMBER BUT ZERO) AND (ANY REAL NUMBER BUT ZERO) evaluates to true (any nonzero number). The AND operator is written && in C++. Do not be confused by thinking it checks equality between numbers: it does not. Keep in mind that the AND operator is evaluated before the OR operator.

OR: Very useful is the OR statement! If either (or both) of the two values it checks are TRUE then it returns TRUE. For example, (1) OR (0) evaluates to 1. (0) OR (0) evaluates to 0. (ANY REAL NUMBER) OR (ANY REAL NUMBER BUT ZERO) evalutes to 1. The OR is written as || in C++. Those are the pipe characters. On your keyboard, they may look like a stretched colon. On my computer the pipe shares its key with \. Keep in mind that OR will be evaluated after AND.

It is possible to combine several boolean operators in a single statement; often you will find doing so to be of great value when creating complex expressions for if statements. What is !(1 && 0)? Of course, it would be TRUE. It is true is because 1 && 0 evaluates to 0 and ! 0 evaluates to a TRUE number (ie, a nonzero number). 

Try some of these - they're not too hard. If you have questions about them, you can email me.

 A. !(1 || 0)          ANSWER: 0 
        B. !(1 || 1 && 0)      ANSWER: 0 (AND is evaluated before OR)
     C. !((1 || 0) && 0)      ANSWER: 1 (Parenthesis are useful)

If you find you enjoyed this section, then you might want to look more at Boolean Algebra.

Lesson 3: Loops
Loops are used to repeat a block of code. You should understand the concept of C++'s true and false, because it will be necessary when working with loops.There are three types of loops: for, while, (and) do while. Each of them has their specific uses. They are all outlined below.

FOR - for loops are the most useful type. The layout is for(variable initialization, conditional expression, modification of variable) The variable initialization allows you to either declare a variable and give it a value or give a value to an already declared variable. Second, the conditional expression tells the program that while the conditional expression is true the loop should continue to repeat itself. The variable modification section is the easiest way for a for loop to handle changing of the variable. It is possible to do things like x++, x=x+10, or even x=random(5);, and if you really wanted to, you could call other functions that do nothing to the variable. That would be something ridiculous probably.

Example:
#include <iostream.h>       //We only need one header file   
int main()        //We always need this
 {                 //The loop goes while x<100, and x increases by one every loop
  for(int x=0;x<100;x++) //Keep in mind that the loop condition checks 
  {    //the conditional statement before it loops again.
    //consequently, when x equals 100 the loop breaks     
    cout<<x<<endl;           //Outputting x 
 } 
  return 0;  
}
This program is a very simple example of a for loop. x is set to zero, while x is less than 100 it calls cout<<x<<endl; and it adds 1 to x until the loop ends. Keep in mind also that the variable is incremented after the code in the loop is run for the first time. WHILE - WHILE loops are very simple. The basic structure is...WHILE(true) then execute all the code in the loop. The true represents a boolean expression which could be x==1 or while(x!=7) (x does not equal 7). It can be any combination of boolean statements that are legal. Even, (while x==5 || v==7) which says execute the code while x equals five or while v equals 7..

Example: 

#include <iostream.h>   //We only need this header file
int main()        //Of course... 
{ 
 int x=0;    //Don't forget to declare variables 
 while(x<100)  //While x is less than 100 do  
 {
    cout<<x<<endl; //Same output as the above loop 
    x++;     //Adds 1 to x every time it repeats, in for loops the  
           //loop structure allows this to be done in the structure  
  }     
 return 0;
}
This was another simple example, but it is longer than the above FOR loop. The easiest way to think of the loop is to think the code executes, when it reaches the brace at the end it goes all the way back up to while, which checks the boolean expression.

DO WHILE - DO WHILE loops are useful for only things that want to loop at least once. The structure is DO {THIS} WHILE (TRUE);

Example: 
#include <iostream.h>
int main()
{
  int x;
  x=0;
  do
  {
    cout<<"Hello world!";
  }while(x!=0);        //Loop while x is not zero, ut first execute the
              //code in the section. (It outputs "Hello..." once
  return 0;
}

Keep in mind that you must include a trailing semi-colon after while in the above example. Notice that this loop will also execute once, because it automatically executes before checking the truth statement. 
Lesson 4: Functions
Now that you should have learned about variables, loops, and if statements it is time to learn about functions. You should have an idea of their uses. Cout is an example of a function. In general, functions perform a number of pre-defined commands to accomplish something productive. 

Functions that a programmer writes will generally require a prototype. Just like an blueprint, the prototype tells the compiler what the function will return, what the function will be called, as well as what arguments the function can be passed. When I say that the function returns a value, I mean that the function can be used in the same manner as a variable would be. For example, a variable can be set equal to a function that returns a value between zero and four. 

For example: 
int a;
a=random(5); //random is sometimes defined by the compiler
//Yes, it returns between 0 and the argument minus 1

Do not think that 'a' will change at random, it will be set to the value returned when the function
is called, but it will not change again. 

The general format for a prototype is simple: 

return-type function_name(arg_type arg); 

There can be more than one argument passed to a function, and it does not have to return a value. Lets look at a function prototype:

int mult(int x, int y);

This prototype specifies that the function mult will accept two arguments, both integers, and that it will return an integer. Do not forget the trailing semi-colon. Without it, the compiler will probably think that you are trying to write the actual definition of the function.

When the programmer actually defines the function, it will begin with the prototype, minus the semi-colon. Then there should always be a bracket (remember, functions require brackets around them) followed by code, just as you would write it for the main function. Finally, end it all with a cherry and a bracket. Okay, maybe not a cherry. 

Lets look at an example program:

#include <iostream.h>

int mult(int x, int y);

int main()
{
  int x, y;
  cout<<"Please input two numbers to be multiplied: ";
  cin>>x>>y;
  cout<<"The product of your two numbers is "<<mult(x, y);
  return 0;
}
int mult(int x, int y)
{
  return x*y;
}

This program begins with the only necessary include file. It is followed by the prototye of the function. Notice that it has the final semi-colon! The main function is an integer, which you should always have, to conform to the standard. You should not have trouble understanding the input and output functions. It is fine to use cin to input to variables as the program does.

Notice how cout actually outputs what appears to be the mult function. What is really happening is that mult acts as a variable. Because it returns a value it is possible for the cout function to output the return value. 

The mult function is actually defined below main. Due to its prototype being above main, the compiler still recognizes it as being defined, and so the compiler will not give an error about mult being undefined, although the definition is below where it is used. 

Return is the keyword used to force the function to return a value. Note that it is possible to have a function that returns no value. In that case, the prototype would have a return type of void. 

The most important functional (Pun semi-intended) question is why. Functions have many uses. For example, a programmer may have a block of code that he has repeated forty times throughout the program. A function to execute that code would save a great deal of space, and it would also make the program more readable. 

Another reason for functions is to break down a complex program into something manageable. For example, take a menu program that runs complex code when a menu choice is selected. The program would probably best be served by making functions for each of the actual menu choices, and then breaking down the complex tasks into smaller, more manageable takes, which could be in their own functions. In this way, a program can be designed that makes sense when read

Lesson 5: switch case

Switch case statements are a substitute for long if statements. The basic format for using switch case is outlined below.

Switch (expression or variable)  
{   
 case variable equals this:   
  do this;  
  break; 
  case variable equals this:   
  do this;  
  break;  
  case variable equals this:  
  do this;  
  break; 
  ...  
  default:  
  do this 
}

The expression or variable has a value. The case says that if it has the value of whatever is after that cases then do whatever follows the colon. The break is used to break out of the case statements. Break is a keyword that breaks out of the code block, usually surrounded by braces, which it is in. In this case, break prevents the program from testing the next case statement also.

Switch case serves as a simple way to write long if statements. Often it can be used to process input from a user. 

Below is a sample program, in which not all of the proper functions are actually declared, but which shows how one would use switch case in a program.

#include <iostream.h> 
#include <conio.h> 
int main()
{    
  int input;   
  cout<<"1. Play game"; 
  cout<<"2. Load game"; 
  cout<<"3. Play multiplayer"; 
  cout<<"4. Exit"; 
  cin>>input;  
  switch (input) 
  {    
   case 1: playgame();
    break;  
   case 2:
   loadgame();  
        break; 
   case 3:       //Note use of : not ;  
        playmultiplayer();
   break; 
   case 4:
        return 0;   
  default:    
        cout<<"Error, bad input, quitting"; 
  }
  return 0;
}
This program will not compile yet, but it serves as a model (albeit simple) for processing input. 

If you do not understand this then try mentally putting in if statements for the case statements. Note that using return in the middle of main will automatically end the program. Default simply skips out of the switch case construction and allows the program to terminate naturally. If you do not like that, then you can make a loop around the whole thing to have it wait for valid input. I know that some functions were not prototyped. You could easily make a few small functions if you wish to test the code.
Lesson 6: An introduction to pointers
Pointers can be confusing, and at times, you may wonder why you would ever want to use them. The truth is, they can make some things much easier. For example, using pointers is one way to have a function modify a variable passed to it; it is also possible to use pointers to dynamically allocate memory allows certain programming techniques, such as linked lists. 
Pointers are what they sound like...pointers. They point to locations in memory. Picture a big jar that holds the location of another jar. In the other jar holds a piece of paper with the number 12 written on it. The jar with the 12 is an integer, and the jar with the memory address of the 12 is a pointer

Pointer syntax can also be confusing, because pointers can both give the memory location and give the actual value stored in that same location. When a pointer is declared, the syntax is this: variable_type *name; Notice the *. This is the key to declaring a pointer, if you use it before the variable name, it will declare the variable to be a pointer. 

As I have said, there are two ways to use the pointer to access information about the memory address it points to. It is possible to have it give the actual address to another variable, or to pass it into a function. To do so, simply use the name of the pointer without the *. However, to access the actual memory location, use the *. The technical name for this doing this is dereferencing. 

In order to have a pointer actually point to another variable it is necessary to have the memory address of that variable also. To get the memory address of the variable, put the & sign in front of the variable name. This makes it give its address. This is called the reference operator, because it returns the memory address. 
For example:
#include <iostream.h>
int main()
{ 
 int x;              //A normal integer 
 int *pointer;    //A pointer to an integer 
 pointer=&x;      //Read it, "pointer equals the address of x"
 cin>>x;          //Reads in x    
 cout<<*pointer; //Note the use of the * to output the actual number stored in x
 return 0;
}  

The cout outputs the value in x. Why is that? Well, look at the code. The integer is called x. A pointer to an integer is then defined as "pointer". Then it stores the memory location of x in pointer by using the ampersand (&) symbol. If you wish, you can think of it as if the jar that had the integer had a ampersand in it then it would output its name (in pointers, the memory address) Then the user inputs the value for x. Then the cout uses the * to put the value stored in the memory location of pointer. If the jar with the name of the other jar in it had a * in front of it would give the value stored in the jar with the same name as the one in the jar with the name. It is not too hard, the * gives the value in the location. The unastricked gives the memory location. 

Notice that in the above example, pointer is initialized to point to a specific memory address before it is used. If this was not the case, it could be pointing to anything. This can lead to extremely unpleasant consequences to the computer. You should always initialize pointers before you use them.

It is also possible to initialize pointers using free memory. This allows dynamic allocation of array memory. It is most useful for setting up structures called linked lists. This difficult topic is too complex for this text. An understanding of the keywords new and delete will, however, be tremendously helpful in the future.

The keyword new is used to initialize pointers with memory from free store (a section of memory available to all programs). The syntax looks like the example:

Example:
int *ptr = new int;

It initializes ptr to point to a memory address of size int (because variables have different sizes, number of bytes, this is necessary). The memory that is pointed to becomes unavailable to other programs. This means that the careful coder will free this memory at the end of its usage.

The delete operator frees up the memory allocated through new. To do so, the syntax is as in the example.

Example:
delete ptr; 

After deleting a pointer, it is a good idea to reset it to point to NULL. NULL is a standard compiler-defined statement that sets the pointer to point to, literally, nothing. By doing this, you minimize the potential for doing something foolish with the pointer. 

The final implication of NULL is that if there is no more free memory, it is possible for the ptr after being "new"-ed to point to NULL. Therefore, it is good programming practice to check to ensure that the pointer points to something before using it. Obviously, the program is unlikely to work without this check.
Lesson 7: Structures 

Before discussing classes, this lesson will be an introduction to data structures similar to classes. Structures are a way of storing many different variables of different types under the same name. This makes it a more modular program, which is easier to modify because its design makes things more compact. It is also useful for databases.

The format for declaring a structure(in C++, it is different in C) is 
struct NAME
{
  VARIABLES;
};
Where NAME is the name of the entire type of structure. To actually create a single structure the syntax is NAME name_of_single_structure; To access a variable of the structure it goes name_of_single_structure.name_of_variable;

For example:
struct example
{
  int x;
};

example an_example;  //Treating it like a normal variable type
an_example.x=33;     //How to access it

Here is an example program: 
struct database
{
  int id_number;
  int age;
  float salary;
};
int main()
{
  database employee;  //There is now an employee variable that has modifiable 
       //variables inside it.
  employee.age=22;
  employee.id_number=1;
  employee.salary=12000.21;
  return 0;
}

The struct database declares that database has three variables in it, age, id_number, and salary.

You can use database like a variable type like int. You can create an employee with the database type as I did above. Then, to modify it you call everything with the 'employee.' in front of it. You can also return structures from functions by defining their return type as a structure type. Example:

struct database fn();

I suppose I should explain unions a little bit. They are like structures except that all the variables share the same memory. When a union is declared the compiler allocates enough memory for the largest data-type in the union. Its like a giant storage chest where you can store one large item, or a bunch of small items, but never the both at the same time.

The '.' operator is used to access different variables inside a union also. 

As a final note, if you wish to have a pointer to a structure, to actually access the information stored inside the structure that is pointed to, you use the -> operator in place of the . operator.

A quick example: 
#include <iostream.h>
struct xampl
{
  int x;
};

int main()
{  
  xampl structure;
  xampl *ptr;
  structure.x=12;
  ptr=&structure; //Yes, you need the & when dealing with structures
      //and using pointers to them
  cout<<ptr->x;  //The -> acts somewhat like the * when used with pointers
   //It says, get whatever is at that memory address
   //Not "get what that memory address is"
  return 0;
}
Lesson 8: Array basics
Arrays are useful critters because they can be used in many ways. For example, a tic-tac-toe board can be held in an array. Arrays are essentially a way to store many values under the same name. You can make an array out of any data-type including structures and classes. 

Think about arrays like this:

[][][][][][] 

Each of the bracket pairs is a slot(element) in the array, and you can put information into each one of them. It is almost like having a group of variables side by side. 
Lets look at the syntax for declaring an array.

int examplearray[100]; //This declares an array

This would make an integer array with 100 slots, or places to store values(also called elements). To access a specific part element of the array, you merely put the array name and, in brackets, an index number. This corresponds to a specific element of the array. The one trick is that the first index number, and thus the first element, is zero, and the last is the number of elements minus one. 0-99 in a 100 element array, for example.

What can you do with this simple knowledge? Lets say you want to store a string, because C++ has no built-in datatype for strings, at least in DOS, you can make an array of characters. 
For example:
char astring[100]; 

will allow you to declare a char array of 100 elements, or slots. Then you can receive input into it it from the user, and if the user types in a long string, it will go in the array. The neat thing is that it is very easy to work with strings in this way, and there is even a header file called string.h. There is another lesson on the uses of string.h, so its not necessary to discuss here. 
The most useful aspect of arrays is multidimensional arrays. 

How I think about multi-dimensional arrays.
[][][][][]
[][][][][]
[][][][][]
[][][][][]
[][][][][]

This is a graphic of what a two-dimensional array looks like when I visualize it.

For example:

int twodimensionalarray[8][8];

declares an array that has two dimensions. Think of it as a chessboard. You can easily use this to store information about some kind of game or to write something like tic-tac-toe. To access it, all you need are two variables, one that goes in the first slot and one that goes in the second slot. You can even make a three dimensional array, though you probably won't need to. In fact, you could make a four-hundred dimensional array. It would be confusing to visualize, however. 
Arrays are treated like any other variable in most ways. You can modify one value in it by 
putting:

arrayname[arrayindexnumber]=whatever; 
//or, for two dimensional arrays
arrayname[arrayindexnumber1][arrayindexnumber2]=whatever;

However, you should never attempt to write data past the last element of the array, such as when
you have a 10 element array, and you try to write to the 11 element. The memory for the array
that was allocated for it will only be ten locations in memory, but the twelfth could be 
anything, which could crash your computer.

You will find lots of useful things to do with arrays, from store information about certain
things under one name, to making games like tic-tac-toe. One suggestion I have is to use for 
loops when access arrays. 

#include <iostream.h>
int main()
{
  int x, y, anarray[8][8];//declares an array like a chessboard
  for(x=0; x<8; x++)
  {
    for(y=0; y<8; y++)
    {
      anarray[x][y]=0;//sets the element to zero; after the loop all elements == 0
    }
  }
  for(x=0; x<8;x++)
  {
    for(y=0; y<8; y++)
    {
cout<<"anarray["<<x<<"]["<<y<<"]="<<anarray[x][y]<<" ";//you'll see
    } 
  }
 return 0;
}

Here you see that the loops work well because they increment the variable for you, and you only
need to increment by one. Its the easiest loop to read, and you access the entire array.

One thing that arrays don't require that other variables do, is a reference operator when
you want to have a pointer to the string. For example:

char *ptr;
char str[40];
ptr=str; //gives the memory address without a reference operator(&)

//As opposed to 

int *ptr;
int num;
ptr=&num;//Requires & to give the memory address to the ptr
Lesson 9: Strings
In C++ strings are really arrays, but there are some different functions that are used for strings, like adding to strings, finding the length of strings, and also of checking to see if strings match. 

The definition of a string would be anything that contains more than one character strung together. For example, "This" is a string. However, single characters will not be strings, though they can be used as strings.

Strings are arrays of chars. Static strings are words surrounded by double quotation marks.

"This is a static string"

To declare a string of 50 letters, you would want to say:

char string[50];


This would declare a string with a length of 50 characters. Do not forget that arrays begin at zero, not 1 for the index number. In addition, a string ends with a null character, literally a '/0' character. However, just remember that there will be an extra character on the end on a string. It is like a period at the end of a sentence, it is not counted as a letter, but it still takes up a space. Technically, in a fifty char array you could only hold 49 letters and one null character at the end to terminate the string.

TAKE NOTE: char *arry; 
Can also be used as a string. If you have read the tutorial on pointers, you can do something such as:

arry = new char[256];

which allows you to access arry just as if it were an array. Keep in mind that to use delete you must put [] between delete and arry to tell it to free all 256 bytes of memory allocated.

For example,
delete [] arry.

Strings are useful for holding all types of long input. If you want the user to input his or her name, you must use a string. 

Using cin>> to input a string works, but it will terminate the string after it reads the first space. The best way to handle this situation is to use the function cin.getline. Technically cin is a class, and you are calling one of its member functions. The most important thing is to understand how to use the function however. 

The prototype for that function is:
cin.getline(char *buffer, int length, char terminal_char);

The char *buffer is a pointer to the first element of the character array, so that it can actually be used to access the array. The int length is simply how long the string to be input can be at its maximum (how big the array is). The char terminal_char means that the string will terminate if the user inputs whatever that character is. Keep in mind that it will discard whatever the terminal character is. 

It is possible to make a function call of cin.getline(arry, '\n'); without the length, or vice versa, cin.getline(arry, 50); without the terminal character. Note that \n is the way of actually telling the compiler you mean a new line, i.e. someone hitting the enter key. 

For a example:

#include <iostream.h>
int main()
{
  char string[256];   //A nice long string
  cout<<"Please enter a long string: ";
  cin.getline(string, 256, '\n'); //The user input goes into string
  cout<<"Your long string was:"<<endl<<string;
  return 0;
}
Remember that you are actually passing the address of the array when you pass string because arrays do not require a reference operator (&) to be used to pass their address. Other than that, you could make \n any character you want (make sure to enclose it with single quotes to inform the compiler of its character status) to have the getline terminate on that character. 

String.h is a header file that contains many functions for manipulating strings. One of these is the string comparison function.

int strcmp(const char *s1, const char *s2);

strcmp will accept two strings. It will return an integer. This integer will either be:
Negative if s1 is less than s2.
Zero if s1 and s2 are equal.
Positive if s1 is greater than s2.

Strcmp is case sensitive. Strcmp also passes the address of the character array to the function to allow it to be accessed.

int strcmpi(const char *s1, const char *s2);

strcmp will accept two strings. It will return an integer. This integer will either be:
Negative if s1 is less than s2.
Zero if the s1 and s2 are equal.
Positive if s1 is greater than s2.

Strcmpi is not case sensitive, if the words are capitalized it does not matter.Not ANSI C++

char *strcat(char *desc, char *src);

strcat is short for string concatenate, which means to add to the end, or append. It adds the second string to the first string. It returns a pointer to the concatenated string. 

char *strupr(char *s);

strupr converts a string to uppercase. It also returns a string, which will all be in uppercase. The input string, if it is an array and not a static string, will also all be uppercase. Not ANSI C++

char *strlwr(char *s);

strlwr converts a string to lowercase. It also returns a string, which will all be in uppercase. The input string, if it is an array, will also all be uppercase.

size_t strlen(const char *s);

strlen will return the length of a string, minus the termating character(/0). The size_t is nothing to worry about. Just treat it as an integer, which it is.


Here is a small program using many of the previously described functions:

#include <iostream.h>   //For cout
#include <string.h>     //For many of the string functions
int main()
{
  char name[50];            //Declare variables
  char lastname[50];        //This could have been declared on the last line...
  cout<<"Please enter your name: ";   //Tell the user what to do
  cin.getline(name, 50, '\n');       //Use gets to input strings with spaces or 
//just to get strings after the user presses enter
  if(!strcmpi("Alexander", name))  //The ! means not, strcmpi returns 0 for
  {                                //equal strings
    cout<<"That's my name too."<<endl; //Tell the user if its my name
  }
  else                              //else is used to keep it from always 
  {      //outputting this line
     cout<<"That's not my name.";  
  } 
   cout<<"What is your name in uppercase..."<<endl; 
  strupr(name);                   //strupr converts the string to uppercase
  cout<<name<<endl;               
  cout<<"And, your name in lowercase..."<<endl;
  strlwr(name);                    //strlwr converts the string to lowercase
  cout<<name<<endl;
  cout<<"Your name is "<<strlen(name)<<" letters long"<<endl;  //strlen returns 
//the length of the string
  cout<<"Enter your last name:";
  cin.getline(lastname, 50, '\n'); //lastname is also a string
  strcat(name, " ");   //We want to space the two names apart
  strcat(name, lastname);      //Now we put them together, we a space in 
//the middle
  cout<<"Your full name is "<<name; //Outputting it all...
  return 0;
}

Lesson 10: C++ File I/O
This is a slightly more advanced topic than what I have covered so far, but I think that it is useful. File I/O is reading from and writing to files. This lesson will only cover text files, that is, files that are composed only of ASCII text. 

C++ has two basic classes to handle files, ifstream and ofstream. To use them, include the header file fstream.h. Ifstream handles file input (reading from files), and ofstream handles file output (writing to files). The way to declare an instance of the ifstream or ofstream class is:

ifstream a_file;
//or
ifstream a_file("filename");

The constructor for both classes will actually open the file if you pass the name as an argument. As well, both classes have an open command (a_file.open()) and a close command (a_file.close()). It is generally a good idea to clean up after yourself and close files once you are finished.

The beauty of the C++ method of handling files rests in the simplicity of the actual functions used in basic input and output operations. Because C++ supports overloading operators, it is possible to use << and >> in front of the instance of the class as if it were cout or cin.

For example: 
#include <fstream.h>
#include <iostream.h>

int main()
{
  char str[10];  
         //Used later
  ofstream a_file("example.txt");
   //Creates an instance of ofstream, and opens example.txt
  a_file<<"This text will now be inside of example.txt"; 
   //Outputs to example.txt through a_file
  a_file.close();                           
   //Closes up the file
   ifstream b_file("example.txt"); 
   //Opens for reading the file
  b_file>>str;    
             //Reads one string from the file
  cout<<str;   
           //Should output 'this'
  b_file.close();    
           //Do not forget this! 
}
The default mode for opening a file with ofstream's constructor is to create it if it does not exist, or delete everything in it if something does exist in it. If necessary, you can give a second argument that specifies how the file should be handled. They are listed below:

ios::app -- Opens the file, and allows additions at the end
ios::ate -- Opens the file, but allows additions anywhere
ios::trunc -- Deletes everything in the file
ios::nocreate -- Does not open if the file must be created
ios::noreplace -- Does not open if the file already exists

For example:
ofstream a_file("test.txt", ios::nocreate);

The above code will only open the file test.txt if that file already exists. 
Lesson 11: Typecasting
Typecasting is making a variable of one type, such as an int, act like another type, a char, for one single application. 

To typecast something, simply put the type of variable you want the actual variable to act as inside parentheses in front of the actual variable. (char)a will make 'a' function as a char.


For example:
#include <iostream.h> 
int main()       
{
  cout<<(char)65; 
  //The (char) is a typecast, telling the computer to interpret the 65 as a
  //character, not as a number.  It is going to give the ASCII output of 
  //the equivalent of the number 65(It should be the letter A).
  return 0;
}

One use for typecasting for is when you want to use the ASCII characters. For example, what if you want to create your own chart of all 256 ASCII characters. To do this, you will need to use to typecast to allow you to print out the integer as its character equivalent.

#include <iostream.h>
int main()
{
  for(int x=0; x<256; x++) 
  {  //The ASCII character set is from 0 to 255
    cout<<x<<". "<<(char)x<<" "; 
  //Note the use of the int version of x to 
        //output a number and the use of (char) to 
  // typecast the x into a character  
        //which outputs the ASCII character that 
  //corresponds to the current number
  }
  return 0;

Lesson 12: Introduction to Classes
C++ is a bunch of small additions to C, and one major addition. This one addition is the object-oriented approach. As its name suggests, this deals with objects. Of course, these are not real-life objects. Instead, these objects are the essential definitions of real world objects. Structures are one step away from these objects, they do not possess one element of them: functions. The definitions of these objects are called classes. The easiest way to think about a class is to imagine a structure that has functions. 

What is this mysterious structure (not the programming type)? Well, it is not only a collection of variables under one heading, but it is a collection of functions under that same heading. If the structure is a house, then the functions will be the doors and the variables will be the items inside the house. They usually will be the only way to modify the variables in this structure, and they are usually the only to access the variables in this structure. 
The idea to make programs more modular. A section of code will have its own functions and variables that control what it can do, and it does not require anything outside of itself to function. While the class may require initialization with data, it does not require outside variables or functions to manipulate the data. That allows programs to reuse the same code more easily. 
From now on, we shall call these structures with functions classes (I guess Marx would not like C++). The syntax for these classes is simple. First, you put the keyword 'class' then the name of the class. Our example will use the name computer. Then you put an open bracket. Before putting down the different variables, it is necessary to put the degree of restriction on the variable. There are three levels of restriction. The first is public, the second protected, and the third private. For now, all you need to know is that the public restriction allows any part of the program, including that which is not part of the class, access the variables specified as public. The protected restriction prevents functions outside the class to access the variable. The syntax for that is merely the restriction keyword (public, private, protected) and then a colon. Finally, you put the different variables and functions (You usually will only put the function prototype[s]) you want to be part of the class. Then you put a closing bracket and semicolon. Keep in mind that you still must end the function prototype(s) with a semi-colon.

Classes should always contain two functions: the constructor and destructor. The syntax for them is simple, the class name denotes a constructor, a ~ before the class name is a destructor. The basic idea is to have the constructor initialize variables, and to have the destructor clean up after the class, which includes freeing any memory allocated. The only time the constructor is called is when the programmer declares an instance of the class, which will automatically call the constructor. The only time the destructor is called is when the instance of the class is no longer needed. When the program ends, or when its memory is deallocated (if you do not understand the deallocation part, do not worry). Keeps in mind this: NEITHER constructors NOR destructors RETURN AN ARGUMENT! This means you do not want to try to return a value in them.

The syntax for defining a function that is a member of a class outside of the actual class definition is to put the return type, then put the class name, two colons, and then the function name. This tells the compiler that the function is a member of that class.

For example:
void Aclass::aFunction()
{
  cout<<"Whatever code";
}


#include <iostream.h>
class Computer //Standard way of defining the class
{
 public: 
 //This means that all of the functions below this(and any variables)
  //are accessible to the rest of the program.
  //NOTE: That is a colon, NOT a semicolon...
Computer();
 //Constructor
 ~Computer();
 //Destructor
 void setspeed(int p);
 int readspeed();
  //These functions will be defined outside of the class
protected:
   //This means that all the variables under this, until a new type of
   //restriction is placed, will only be accessible to other functions in the
   //class.  NOTE: That is a colon, NOT a semicolon...
int processorspeed;
};  
   //Do Not forget the trailing semi-colon

Computer::Computer() 
{ //Constructors can accept arguments, but this one does not
  processorspeed = 0; 
  //Initializes it to zero
}

Computer::~Computer()
{  //Destructors do not accept arguments
} 
//The destructor does not need to do anything.

void Computer::setspeed(int p) 
{       //To define a function outside put the name of the function 
 //after the return type and then two colons, and then the name 
 //of the function. 
  processorspeed = p;
}
int Computer::readspeed()  
{ //The two colons simply tell the compiler that the function is part
 //of the clas
  return processorspeed;
}
int main()
{
  Computer compute;  
  //To create an 'instance' of the class, simply treat it like you would
    //a structure.  (An instance is simply when you create an actual object
  //from the class, as opposed to having the definition of the class)
  compute.setspeed(100); 
  //To call functions in the class, you put the name of the instance,
    //a period, and then the function name.
  cout<<compute.readspeed();
  //See above note.
  return 0;
}

As you can see, this is a rather simple concept. However, it is very powerful. It makes it easy to prevent variables that are contained (or owned) by the class being overwritten accidentally. It also allows a totally different way of thinking about programming. I want to end this tutorial as an introduction, however. 
Lesson 13: More on Functions 
In lesson 4 you were given the basic information on functions. However, I left out two items of interest. First, when you declare a function you do not have to prototype it! You must give the function definition physically before you call the function. You simply type in the entire definition of the function where you would normally put the prototype. 

For example:
#include <iostream.h>
void function(void)  
{      //Normally this would be the prototype. Do not include a semicolon
  //Only prototypes have semicolons
  cout<<"HA!  NO PROTOTYPE!";
}

int main()
{
  function();
 //It works like a normal function now.
 return 0;
}
The other programming concept is the inline function. Inline functions are not very important, but it is good to understand them. The basic idea is to save time at a cost in space. Inline functions are a lot like a placeholder. Once you define an inline function, using the 'inline' keyword, whenever you call that function the compiler will replace the function call with the actual code from the function. 
How does this make the program go faster? Simple, function calls are simply more time consuming than writing all of the code without functions. To go through your program and replace a function you have used 100 times with the code from the function would be time consuming. Of course, by using the inline function to replace the function calls with code you will also greatly increase the size of your program. 

Using the inline keyword is simple, just put it before the name of a function. Then, when you use that function, pretend it is a non-inline function. 

For example:
#include <iostream.h>
inline void hello(void)
{  //Just use the inline keyword before the function    
   cout<<"hello";
}
int main()
{
 hello();
   //Call it like a normal function...
  return 0;
}

However, once the program is compiled, the call to hello(); will be replaced by the code making up the function. 

A WORD OF WARNING: Inline functions are very good for saving time, but if you use them too often or with large functions you will have a tremendously large program. Sometimes large programs are actually less efficient, and therefore they will run more slowly than before. Inline functions are best for small functions that are called often. 

In the future, we will discuss inline functions in terms of C++ classes. However, now that you understand the concept I will feel comfortable using inline functions in later tutorials. 
Lesson 14: Accepting command line arguments
In C++ it is possible to accept command line arguments. To do so, you must first understand the full definition of int main(). It actually accepts two arguments, one is number of command line arguments, the other is a listing of the command line arguments. 

It looks like this:

int main(int argc, char* argv[])

The interger, argc is the ARGument Count (hence argc). It is the number of arguments passed into the program from the command line, including the path to and name of the program. 

The array of character pointers is the listing of all the arguments. argv[0] is entire path to the program including its name. After that, every element number less than argc are command line arguments. You can use each argv element just like a string, or use argv as a two dimensional array.

How could this be used? Almost any program that wants it parameters to be set when it is executed would use this. One common use is to write a function that takes the name of a file and outputs the entire text of it onto the screen.

#include <fstream.h> //Needed to manipulate files
#include <iostream.h>
#include <io.h>      //Used to check file existence
int main(int argc, char * argv[])
{
  if(argc!=2)
  {
    cout<<"Correct input is: filename"; 
    return 0;   
  }             
  if(access(argv[1], 00)) //access returns 0 if the file can be accessed 
  {     //under the specified method (00)
    cout<<"File does not exist";  //because it checks file existence
    return 0;
  }
  ifstream the_file;  //ifstream is used for file input
  the_file.open(argv[1]); //argv[1] is the second argument passed in 
    //presumably the file name 
  char x;
  the_file.get(x);  
  while(x!=EOF)  //EOF is defined as the end of the file
  {
    cout<<x;
    the_file.get(x);//Notice we always let the loop check x for the end of   
  }  //file to avoid bad output when it is reached
  the_file.close();  //Always clean up
  return 0;
} 

This program is fairly simple. It incorporates the full version of main. Then it first checks to ensure the user added the second argument, theoretically a file name. It checks this, using the access function, which accepts a file name and an access type, with 00 being a check for existence. This is not a standard C++ function. It may not work on your compiler Then it creates an instance of the file input class,and it opens the second argument passed into main. If you have not seen get before, it is a standard function used in input and output that is used to input a single character into the character passed to it, or by returning the character. EOF is simply the end of file marker, and x is checked to make sure that the next output is not bad.
Lesson 15: Singly linked lists
Linked lists are a way to store data with structures so that the programmer can automatically create a new place to store data whenever necessary. Specifically, the programmer writes a struct or class definition that contains variables holding information about something, and then has a pointer to a struct of its type. Each of these individual struct or classes in the list is known as a node.

Think of it like a train. The programmer always stores the first node of the list. This would be the engine of the train. The pointer is the connector between cars of the train. Every time the train ads a car, it uses the connectors to add a new car. This is like a programmer using the keyword new to create a pointer to a new struct or class. 
In memory it is often described as looking like this:

----------        ----------
- Data   -       >- Data   -    
----------     -  ----------   
- Pointer- - -    - Pointer-  
----------        ----------

Each of the big blocks is a struct (or class) that has a pointer to another one. Remember that the pointer only stores the memory location of something, it is not that thing, so the arrow goes to the next one. At the end, there is nothing for the pointer to point to, so it does not point to anything, it should be set to "NULL" to prevent it from accidentally pointing to a totally arbitrary and random location in memory (which is very bad). 

So far we know what the node struct should look like: 
struct node
{
  int x;
  node *next;
};
int main()
{
  node *root;  //This will be the unchanging first node
  root=new node; //Now root points to a node struct
  root->next=NULL; //The node root points to has its next pointer
       //set equal to NULL
  root->x=5;   //By using the -> operator, you can modify the node
  return 0;    //a struct (root in this case) points to.
}    
This so far is not very useful for doing anything. It is necessary to understand how to traverse (go through) the linked list before going further. 

Think back to the train. Lets imagine a conductor who can only enter the train through the engine, and can walk through the train down the line as long as the connector connects to another car. This is how the program will traverse the linked list. The conductor will be a pointer to node, and it will first point to root, and then, if the root's pointer to the next node is pointing to something, the "conductor" (not a technical term) will be set to point to the next node. In this fashion, the list can be traversed. Now, as long as there is a pointer to something, the traversal will continue. Once it reaches a NULL pointer, meaning there are no more nodes (train cars) then it will be at the end of the list, and a new node can subsequently be added if so desired.

Here's what that looks like: 
struct node
{
  int x;
  node *next;
};
int main()
{
  node *root; //This won't change, or we would lose the list in memory
  node *conductor; //This will point to each node as it traverses
       //the list
  root=new node; //Sets it to actually point to something
  root->next=NULL; //Otherwise it would not work well
  root->x=12;
  conductor=root; //The conductor points to the first node
  if(conductor!=NULL)
  {
    while(conductor->next!=NULL)
    {
      conductor=conductor->next;   
    }
  }
  conductor->next=new node; //Creates a node at the end of the list
  conductor=conductor->next; //Points to that node
  conductor->next=NULL; //Prevents it from going any further
  conductor->x=42;
}
That is the basic code for traversing a list. The if statement ensures that there is something to begin with (a first node). In the example it will always be so, but if it was changed, it might not be true. If the if statement is true, then it is okay to try and access the node pointed to by conductor. The while loop will continue as long as there is another pointer in the next. The conductor simply moves along. It changes what it points to by getting the address of conductor->next. 

Finally, the code at the end can be used to add a new node to the end. Once the while loop as finished, the conductor will point to the last node in the array. (Remember the conductor of the train will move on until there is nothing to move on to? It works the same way in the while loop.) Therefore, conductor->next is set to null, so it is okay to allocate a new area of memory for it to point to. Then the conductor traverses one more element(like a train conductor moving on the the newly added car) and makes sure that it has its pointer to next set to NULL so that the list has an end. The NULL functions like a period, it means there is no more beyond. Finally, the new node has its x value set. (It can be set through user input. I simply wrote in the '=42' as an example.) 

To print a linked list, the traversal function is almost the same. It is necessary to ensure that the last element is printed after the while loop terminates. 

For example:
conductor=root;
if(conductor!=NULL) //Makes sure there is a place to start
{
  while(conductor->next!=NULL)
  {
    cout<<conductor->x;
    conductor=conductor->next;
  }
  cout<<conductor->x;
}
The final output is necessary because the while loop 

Lesson 16: Recursion
Recursion is defined as a function calling itself. It is in some ways similar to a loop because it repeats the same code, but it requires passing in the looping variable and being more careful. Many programming languages allow it because it can simplify some tasks, an it is often more elegant than a loop. 

A simple example of recursion would be: 
void recurse()
{
  recurse(); //Function calls itself
}
int main()
{
  recurse(); //Sets off the recursion
  return 0;  //Rather pitiful, it will never be reached
}
This program will not continue forever, however. The computer keeps function calls on a stack and once too many are called without ending, the program will terminate. Why not write a program to see how many times the function is called before the program terminates? 

#include <iostream.h>
void recurse(int count) //The count variable is initalized by each function call
{
  cout<<count;  
  recurse(count+1); //It is not necessary to increment count
//each function's variables
} //are separate (so each count will be initialized one greater)
int main()
{
  recurse(1);        //First function call, so it starts at one
  return 0;          
}

This simple program will show the number of times the recurse function has been called by initializing each individual function call's count variable one greater than it was previous by passing in count+1. Keep in mind, it is not a function restarting itself, it is hundreds of functions that are each unfinished with the last one calling a new recurse function. 

It can be thought of like those little chinese dolls that always have a smaller doll inside. Each doll calls another doll, and you can think of the size being a counter variable that is being decremented by one. 

Think of a really tiny doll, the size of a few atoms. You can't get any smaller than that, so there are no more dolls. Normally, a recursive function will have a variable that performs a similar action; one that controls when the function will finally exit. The condition where the functin will not call itself is termed the base case of the function. Basically, it is an if-statement that checks some variable for a condition (such as a number being less than zero, or greater than some other number) and if that condition is true, it will not allow the function to call itself again. (Or, it could check if a certain condition is true and only then allow the function to call itself). 

A quick example:
void doll(int size)
{
  if(size==0)//No doll can be smaller than 1 atom (10^0==1) so doesn't call itself
    return;    //Return does not have to return something, it can be used 
    //to exit a function
  doll(size-1);  //Decrements the size variable so the next doll will be smaller.
}
int main()
{
  doll(10);   //Starts off with a large doll (its a logarithmic scale)
  return 0;   //Finally, it will be used
}

This program ends when size equals one. This is a good base case, but if it is not properly set up, it is possible to have an base case that is always true (or always false). 

Once a function has called itself, it will be ready to go to the next line after the call. It can still perform operations. One function you could write could print out the numbers 123456789987654321. How can you use recursion to write a function to do this? Simply have it keep incrementing a variable passed in, and then output the variable...twice, once before the function recurses, and once after... 

void printnum(int begin)
{
  cout<<begin;
  if(begin<9)  //The base case is when begin is greater than 9
    printnum(begin+1);    //for it will not recurse after the if-statement
  cout<<begin;  //Outputs the second begin, after the program has 
      //gone through and output
}  //the numbers from begin to 9.

This function works because it will go through and print the numbers begin to 9, and then as each printnum function terminates it will continue printing the value of begin in each function from 9 to begin. 

This is just the beginning of the usefulness of recursion. Heres a little challenge, use recursion to write a program that returns the factorial of any number greater than 0. (Factorial is number*number-1*number-2...*1). 

Hint: Recursively find the factorial of the smaller numbers first, ie, it takes a number, finds the factorial of the previous number, and multiplies the number times that factorial...have fun, email me at webmaster@cprogramming.com if you get it. 

Lesson 17: Functions with variable-length argument lists


Perhaps you would like to have a function that will accept any number of values and then return the average. You don't know how many arguments will be passed in to the function. One way you could make the function would be to accept a pointer to an array. Another way would be to write a function that can take any number of arguments. So you could write avg(4, 12.2, 23.3, 33.3, 12.1); or you could write avg(2, 2.3, 34.4); Some library functions can accept a variable list of arguments (such as the venerable printf). 

To use a function with variable number of arguments, or more precisely, a function without a set number of arguments, you would use the stdarg.h header file. There are four parts needed: va_list, which stores the list of arguments, va_start, which initializes the list, va_arg, which returns the next argument in the list, and va_end, which cleans up the variable argument list. Whenever a function is declared to have an indeterminate number of arguments, in place of the last argument you should place an ellipsis (which looks like '...'), so, int a_function(int x, ...); would tell the compiler the function should accept however many arguments that the programmer uses, as long as it is equal to at least one, the one being the first, x. 

va_list is like any other variable. For example, va_list a_list; 

va_start is a macro which accepts two arguments, a va_list and the name of the variable that directly precedes the ellipsis (...). So, in the function a_function, to initialize a_list with va_start, you would write va_start(a_list, x); 

va_arg takes a va_list and a variable type, and returns the next argument in the list in the form of whatever variable type it is told. It then moves down the list to the next argument. For example, va_arg(a_list, double) will return the next argument, assuming it exists, in the form of a double. The next time it is called, it will return the argument following the last returned number, if one exists. 

To show how each of the parts works, take an example function: 


#include <stdarg.h>

#include <iostream.h>

double average(int num, ...)
{
  va_list arguments;        //A place to store the list of arguments
  va_start(arguments, num); //Initializing arguments to store all values
  int sum=0;       // passed in after num    
  for(int x=0; x<num; x++)   //Loop until all numbers are added
    sum+=va_arg(arguments, double); //Adds the next value in argument list to sum.
  va_end(arguments);      //Cleans up the list
  return sum/(double)num;    //Returns some number (typecast prevents trunctation)

}



int main()

{

  cout<<average(3, 12.2, 22.3, 4.5)<<endl;
  cout<<average(5, 3.3, 2.2, 1.1, 5.5, 3.3)<<endl;
  return 0;

}


It isn't necessarily a good idea to use a variable argument list at all times, because the potential exists for assuming a value is of one type, while it is in fact another, such as a NULL pointer being assumed to be an integer. Consequently, variable argument lists should be used sparingly. 

Binary Trees: Part 1
The binary tree is a fundamental data structure used in computer science. The binary tree is a useful data structure for rapidly storing sorted data and rapidly retrieving stored data. A binary tree is composed of parent nodes, or leaves, each of which stores data and also links to up to two other child nodes (leaves) which can be visualized spatially as below the first node with one placed to the left and with one placed to the right. It is the relationship between the leaves linked to and the linking leaf, also known as the parent node, which makes the binary tree such an efficient data structure. It is the leaf on the left which has a lesser key value (ie, the value used to search for a leaf in the tree), and it is the leaf on the right which has an equal or greater key value. As a result, the leaves on the farthest left of the tree have the lowest values, whereas the leaves on the right of the tree have the greatest values. More importantly, as each leaf connects to two other leaves, it is the beginning of a new, smaller, binary tree. Due to this nature, it is possible to easily access and insert data in a binary tree using search and insert functions recursively called on successive leaves.
The typical graphical representation of a binary tree is essentially that of an upside down tree. It begins with a root node, which contains the original key value. The root node has two child nodes; each child node might have its own child nodes. Ideally, the tree would be structured so that it is a perfectly balanced tree, with each node having the same number of child nodes to its left and to its right. A perfectly balanced tree allows for the fastest average insertion of data or retrieval of data. The worst case scenario is a tree in which each node only has one child node, so it becomes as if it were a linked list in terms of speed. The typical representation of a binary tree looks like the following:
   
             10
           /    \
          6      14
         / \    /  \
        5   8  11  18
The node storing the 10, represented here merely as 10, is the root node, linking to the left and right child nodes, with the left node storing a lower value than the parent node, and the node on the right storing a greater value than the parent node. Notice that if one removed the root node and the right child nodes, that the node storing the value 6 would be the equivalent a new, smaller, binary tree.
The structure of a binary tree makes the insertion and search functions simple to implement using recursion. In fact, the two insertion and search functions are also both very similar. To insert data into a binary tree involves a function searching for an unused node in the proper position in the tree in which to insert the key value. The insert function is generally a recursive function that continues moving down the levels of a binary tree until there is an unused leaf in a position which follows the rules of placing nodes. The rules are that a lower value should be to the left of the node, and a greater or equal value should be to the right. Following the rules, an insert function should check each node to see if it is empty, if so, it would insert the data to be stored along with the key value (in most implementations, an empty node will simply be a NULL pointer from a parent node, so the function would also have to create the node). If the node is filled already, the insert function should check to see if the key value to be inserted is less than the key value of the current node, and if so, the insert function should be recursively called on the left child node, or if the key value to be inserted is greater than or equal to the key value of the current node the insert functin should be recursively called on the right child node. The search function works along a similar fashion. It should check to see if the key value of the current node is the value to be searched. If not, it should check to see if the value to be searched for is less than the value of the node, in which case it should be recursively called on the left child node, or if it is greater than the value of the node, it should be recursively called on the right child node. Of course, it is also necessary to check to ensure that the left or right child node actually exists before calling the function on the node.
Because binary trees have log (base 2) n layers, the average search time for a binary tree is log (base 2) n. To fill an entire binary tree, sorted, takes roughly log (base 2) n * n. Lets take a look at the necessary code for a simple implementation of a binary tree. First, it is necessary to have a struct, or class, defined as a node.

struct node
{
  int key_value;
  node *left;
  node *right;
};

The struct has the ability to store the key_value and contains the two child nodes which define the node as part of a tree. In fact, the node itself is very similar to the node in a linked list. A basic knowledge of the code for a linked list will be very helpful in understanding the techniques of binary trees. Essentially, pointers are necessary to allow the arbitrary creation of new nodes in the tree.
It is most logical to create a binary tree class to encapsulate the workings of the tree into a single area, and also making it reusable. The class will contain functions to insert data into the tree and to search for data. Due to the use of pointers, it will be necessary to include a function to delete the tree in order to conserve memory after the program has finished.
 
class btree
{
  node *root;
  btree();
  ~btree();
  void destroy_tree(node *leaf);
  void insert(int key, node *leaf);
  node *search(int key, node *leaf);
  public:
  void insert(int key);
  node *search(int key);
  void destroy_tree();
};

The insert and search functions that are public members of the class are designed to allow the user of the class to use the class without dealing with the underlying design. The insert and search functions which will be called recursively are the ones which contain two parameters, allowing them to travel down the tree. The destroy_tree function without arguments is a front for the destroy_tree function which will recursively destroy the tree, node by node, from the bottom up. 
The code for the class would look similar to the following:
btree::btree()
{
  root=NULL;
}

It is necessary to initialize root to NULL for the later functions to be able to recognize that it does not exist.
btree::~btree()
{
  destroy_tree();
}
The destroy_tree function will set off the recursive function destroy_tree shown below which will actually delete all nodes of the tree.
void destroy_tree(node *leaf)
{
  if(leaf!=NULL)
  {
    destroy_tree(leaf->left);
    destroy_tree(leaf->right);
    delete leaf;
  }
}
The function destroy_tree goes to the bottom of each part of the tree, that is, searching while there is a non-null node, deletes that leaf, and then it works its way back up. The function deletes the leftmost node, then the right child node from the leftmost node's parent node, then it deletes the parent node, then works its way back to deleting the other child node of the parent of the node it just deleted, and it continues this deletion working its way up to the node of the tree upon which delete_tree was originally called. In the example tree above, the order of deletion of nodes would be 5 8 6 11 18 14 10. Note that it is necessary to delete all the child nodes to avoid wasting memory.
void btree::insert(int key, node *leaf)
{
  if(key< leaf->key_value)
  {
    if(leaf->left!=NULL)
     insert(key, leaf->left);
    else
    {
      leaf->left=new node;
      leaf->left->key_value=key;
      leaf->left->left=NULL;    //Sets the left child of the child node to null
      leaf->left->right=NULL;   //Sets the right child of the child node to null
    }  
  }
  else if(key>=leaf->key_value)
  {
    if(leaf->right!=NULL)
      insert(key, leaf->right);
    else
    {
      leaf->right=new node;
      leaf->right->key_value=key;
      leaf->right->left=NULL;  //Sets the left child of the child node to null
      leaf->right->right=NULL; //Sets the right child of the child node to null
    }
  }
}
The case where the root node is still NULL will be taken care of by the insert function that is nonrecursive and available to non-members of the class. The insert function searches, moving down the tree of children nodes, following the prescribed rules, left for a lower value to be inserted and right for a greater value, until it finds an empty node which it creates using the 'new' keyword and initializes with the key value while setting the new node's child node pointers to NULL. After creating the new node, the insert function will no longer call itself.
node *btree::search(int key, node *leaf)
{
  if(leaf!=NULL)
  {
    if(key==leaf->key_value)
      return leaf;
    if(key<leaf->key_value)
      return search(key, leaf->left);
    else
      return search(key, leaf->right);
  }
  else return NULL;
}
The search function shown above recursively moves down the tree until it either reaches a node with a key value equal to the value for which the function is searching or until the function reaches an uninitialized node, meaning that the value being searched for is not stored in the binary tree. It returns a pointer to the node to the previous instance of the function which called it, handing the pointer back up to the search function accessible outside the class.
void btree::insert(int key)
{
  if(root!=NULL)
    insert(key, root);
  else
  {
    root=new node;
    root->key_value=key;
    root->left=NULL;
    root->right=NULL;
  }
}
The public version of the insert function takes care of the case where the root has not been initialized by allocating the memory for it and setting both child nodes to NULL and setting the key_value to the value to be inserted. If the root node already exists, insert is called with the root node as the initial node of the function, and the recursive insert function takes over.
node *btree::search(int key)
{
  return search(key, root);
}
The public version of the search function is used to set off the search recursion at the root node, keeping it from being necessary for the user to have access to the root node.
void btree::destroy_tree()
{
  destroy_tree(root);
}
The public version of the destroy tree function is merely used to initialize the recursive destroy_tree function which then deletes all the nodes of the tree.

Lesson 19: Inheritance - An Overview
The ability to use the object-oriented programming is an important feature of C++. Lesson 12 introduced the idea of the class; if you have not read it and do not know the basic details of classes, you should read it before continuing this tutorial. This tutorial is n Inheritance is an important feature of classes; in fact, it is integral to the idea of object oriented programming. Inheritance allows you to create a hierarchy of classes, with various classes of more specific natures inheriting the general aspects of more generalized classes. In this way, it is possible to structure a program starting with abstract ideas that are then implemented by specific classes. For example, you might have a class Animal from which class dog and cat inherent the traits that are general to all animals; at the same time, each of those classes will have attributes specific to the animal dog or cat.

Inheritence offers many useful features to programmers. The ability, for example, of a variable of a more general class to function as any of the more specific classes which inherit from it, called polymorphism, is handy. For now, we will concentrate on the basic syntax of inheritance. Polymorphism will be covered in its own tutorial.

Any class can inherit from any other class, but it is not necessarily good practice to use inheritance (put it in the bank rather than go on a vacation). Inheritance should be used when you have a more general class of objects that describes a set of objects. The features of every element of that set (of every object that is also of the more general type) should be reflected in the more general class. This class is called the base class. base classes usually contain functions that all the classes inheriting from it, known as derrived classes, will need. base classes should also have all the variables that every derrived class would otherwise contain.

Let us look at an example of how to structure a program with several classes. Take a program used to simulate the interaction between types of organisms, trees, birds, bears, and other creatures coinhabiting a forest. There would likely be several base classes that would then have derrived classes specific to individual animal types. In fact, if you know anything about biology, you might wish to structure your classes to take advantage of the biological classification from Kingdom to species, although it would probably be overly complex. Instead, you might have base classes for the animals and the plants. If you wanted to use more base classes (a class can be both a derrived of one class and a base of another), you might have classes for flying animals and land animals, and perhaps trees and scrub. Then you would want classes for specific types of animals: pigeons and vultures, bears and lions, and specific types of plants: oak and pine, grass and flower. These are unlikely to live together in the same area, but the idea is essentially there: more specific classes ought to inherit from less specific classes.

Classes, of course, share data. A derrived class has access to most of the functions and variables of the base class. There are, however, ways to keep derrivedren from accessing some attributes of its base class. The keywords public, protected, and private are used to control access to information within a class. It is important to remember that public, protected, and private control information both for specific instances of classes and for classes as general data types. Variables and functions designated public are both inheritable by derrived classes and accessible to outside functions and code when they are elements of a specific instance of a class. Protected variables are not accessible by functions and code outside the class, but derrived classes inherit these functions and variables as part of their own class. Private variables are neither accessible outside the class when it is a specific class nor are available to derrived classes. Private variables are useful when you have variables that make sense in the context of large idea.

Lesson 20: Inheritance - Syntax 
Before beginning this lesson, you should have an understanding of the idea of inheritance. If you do not, please read lesson 19. This lesson will consist of an overview of the syntax of inheritance, the use of the keywords public, private, and protected, and then an example program following to demonstrate each. The syntax to denote one class as inheriting from another is simple. It looks like the following: class Bear : public Animal, in place of simply the keyword class and then the class name. The ": public base_class_name" is the essential syntax of inheritance; the function of this syntax is that the class will contain all public and protected variables of the base class. Do not confuse the idea of a derrived class having access to data members of a base class and specific instances of the derrived class posessing data. The data members - variables and functions - possessed by the derrived class are specific to the type of class, not to each individual object of that type. So, two different Bear objects, while having the same member variables and functions, may have different information stored in their variables; furthermore, if there is a class Animal with an object, say object BigAnimal, of that type, and not of a more specific type inherited from that class, those two bears will not have access to the data within BigAnimal. They will simply possess variables and functions with the same name and of the same type. A quick example of inheritance:
class Animal
{
  public:
  int legs;
  int arms;
  int age;
  Animal();
  ~Animal();
  void eat();
  void sleep();
  void drink();
};
//The class Animal contains information and functions
//related to all animals (at least, all animals this lesson uses)

class Cat : public Animal
{
  public:
  int fur_color;
  void Purr();
  void fish();
  void Mark_territory();
};
//For those of you familiar with cats
//eat of the above operations is unique
//to your friendly furry friends
//(or enemies, as the case may be)

A discussion of the keywords public, private, and protected is useful when discussing inheritance. The three keywords are used to control access to functions and variables stored within a class. 
public: 
The most open level of data hiding, anything that is public is available to all derrived classes of a base class, and the public variables and data for each object of both the base and derived class is accessible by code outside the class. Functions marked public are generally those the class uses to give information to and take information from the outside world; they are typically the interface with the class. The rest of the class should be hidden from the user (This hidden nature and the highly focused nature of classes is known collectively as encapsulation). The syntax for public is: 
public:
Everything following is public until the end of the class or another data hiding keyword is used. 
protected:
Variables and functions marked protected are inherited by derrived classes; however, these derrived classes hide the data from code outside of any instance of the object. Keep in mind, even if you have another object of the same type as your first object, the second object cannot access a protected variable in the first object. Instead, the second object will have its own variable with the same name - but not necessarily the same data. Protected is a useful level of protection for important aspects to a class that must be passed on without allowing it to be accessed. The syntax is the same as that of public. specifically, 
 protected: 
private:
Private is the highest level of data-hiding. Not only are the functions and variables marked private not accessible by code outside the specific object in which that data appears, but private variables and functions are not inherited. The level of data protection afforded by protected is generally more flexible than that of the private level. Of course, there is a certain joy in protecting your data with the keyword private. The syntax remains the same. 

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