Loops

While(Loops)
Introduction:- 
Why Is Repetition Needed? 

Repetition allows you to efficiently use variables. Can input, add, and average multiple numbers using a limited number of variables. For example, to add five numbers: Declare a variable for each number, input the numbers and add the variables together. Create a loop that reads a number into a variable and adds it to a variable that contains the sum of the numbers.

                 While Loop(Repetition) Structure:-
The general form of the while statement is:                                        
 

while is a reserved word.

Statement can be simple or compound. 

Expression acts as a decision maker and is usually a logical expression.

While is a pre-test loop.

Statement is called the body of the loop.

The parentheses are part of the syntax .

Infinite loop: continues to execute endlessly. Avoided by including statements in loop body that assure exit condition is eventually false.

                      
                  Case 1: Counter-Controlled while Loops                  

If you know exactly how many pieces of data need to be read, while loop becomes a counter-controlled loop with loop control variable (LCV) 

#include<iostream.h> 

int main() 

{ 

     int total, //sum of grades

     gradeCounter, //number of grades entered 

     grade, //one grade 

     average; //average of grades

                   //initialization phase 

     total = 0; //clear total 

     gradeCounter = 1; //prepare to loop

                  //processing phase 

     while (gradeCounter <= 10) //loop 10 times 

    { 

          cout << "Enter grade: "; //prompt for input

          cin >> grade; //input grade 

          total = total + grade; //add grade total 

          gradeCounter = gradeCounter +1; //increment counter 

    }

                   //termination phase

    average = total/10; //integer division 

    cout << "Class average is " << average <<endl; 

    return 0; //indicate program ended successfully 

}

                         Case 2: Sentinel-Controlled while Loops

Sentinel variable is tested in the condition

Loop ends when sentinel is encountered

            while(score!=-1)

            {

                 cout<<"Good Bye "<<endl;

                 cin>>score;

            }

                            Case 3: Flag-Controlled while loops                           

A flag-controlled while loop uses a bool variable to control the loop.

The flag-controlled while loop takes the form:

boolean done=false;

while(!done)

{

//read some input

   if(condition)

      done=true;

   else

   {

//process input

   }

 }

         do...while Looping (Repetition) Structure  

General form of a do...while:

do

{

statements;

} 

while(condition)

The statement executes first, and then the expression is evaluated.

To avoid an infinite loop, body must contain a statement that makes the expression false.

The statement can be simple or compound.

This loop is post-test loop.

Loop always iterates at least once.

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                               For(Loops)
for Looping (Repetition) Structure: -

The general form of the for statement is:

for(initialization;test condition;increment)

{

//Body of loop

} 

Example:-

for(int x=0;x<=10;x++)

{

cout<<x;

}

The initial statement,loop condition, and update statement are called for loop control statements.

Initial statement usually initializes a variable (called the for loop control, or  indexed, variable)

In C++, for is a reserved word. If test is false the first time it is evaluated, the body of the loop will not be executed.The any  update expression can increment or decrements by amount.Variables used in the initialization section should not be modified in the body of the loop.
int sum = 0, num;
for (num = 1; num <= 10; num++)
{   
sum += num;
cout << "Sum of numbers 1 – 10 is ";
cout<< sum << endl;
} 
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