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   Lesson 3: Loops

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    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.
       
Ex.


#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 x value before it
    {}            //Actually repeats, or loops, again. So if x==100 the loop will end. 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..

Ex.


#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);

For example:

#include <iostream.h>

int main()
{
int x;
x=0;
do
{
cout<<"Hello world!";
}while(x!=0);
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
.


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