Q4: Explain Looping Concept. Write Syntax of all three looping statements.

Loops:  In real life, there are situations when there is need to perform a set of task repeatedly till some condition is met. Such as – sending email to all employees, deleting all files, printing 1000 pages of a document. All of these tasks are performed in loop.

There are three kind of looping in C Language:

a.    While  Loop, 

b.    Do  While,   

c.    For  Loop. 

For Loop

Parts of for loop

Any repetition contains two important part - What to repeat and number of repetition? Variable-initialization, condition and variable-update define number of repetition and body of loop defines what to repeat.

·         Variable-initialization contain loop counter variable initialization statements. It define starting point of the repetition (where to start loop).

·         Condition contain Boolean expressions and works like if...else. If boolean expression is true, then execute body of loop otherwise terminate the loop.

·         Body of loop specifies what to repeat. It contain set of statements to repeat.

·         Variable-update contains loop counter update (increment/decrement) statements.

How for loop works?

·         Initially variable-initialization block receive program control. It is non-repeatable part and executed only once throughout the execution of for loop. After initialization program control is transferred to loop condition.

·         The loop condition block evaluates all Boolean expression and determines loop should continue or not. If loop conditions are met, then it transfers program control to body of loop otherwise terminate the loop. In C we specify a boolean expression using relational and logical operator.

·         Body of loop executes a set of statements. After executing all statements it transfer program control to variable-update block.

·         Variable-update block updates loop counter variable and transfer program control again back to condition block of loop.

Step 2 to 4 is repeated until condition is met.

Control Flow of for loop

#include <stdio.h> void main() {     int count;     for(count=1; count<=10; count++)     {         printf("%d ", count);     } }

Syntax of for loop

for(variable-initialization ; condition ; variable-update)

{

    // Body of for loop

 

While Loop

for loop is easy to implement if you specifically know start and end position of the loop counter. However, things in the real life are not so simple. You may come across situation where you only know when to terminate the loop. For example – reading instructions from user until terminated manually, waiting for client connection until connected or cancelled, reconnecting to the server until connected.

while loop is an entry controlled looping construct. We use while loop to repeat set of statements when number of iterations are not known prior to its execution. It provides flexibility to define loop without initialization and update parts (present in for loop).

Syntax of while loop while(condition) {     // Body of while loop }

Control-Flow

#include <stdio.h>   void main() {     int n = 1;     /* Loop condition */     while(n <= 5)     {         /* Body of loop */         printf("%d ", n);         /* Update loop counter variable */         n = n + 1;     } }

 

Do-while Loop

·        C programming supports three types of looping statements for loop, while loop and do...while loop. Among three do...while loop is most distinct loop compared to others.

·        do...while is an exit controlled looping statement.

·        do...while loop is used, when there is a need to check condition after execution of loop body.

·        do...while loop in any case executes minimum once.

·        Looping statements whose condition is checked after execution of its loop body is called as Exit controlled loop

·        For example - consider a program to validate user input and run in loop until user feeds valid input. In this case the input statement should run minimum once and should repeat in loop until user provides valid input.

Flow Control

Syntax of do...while loop do {     // Body of do while loop } while (condition);

 

How do...while loop works?

do...while loop works in two step.

1.    Initially program control transfers to body of loop. It executes all statements inside loop body and transfers control to loop condition.

2.    Loop condition contains set of relational and logical expressions. If conditional expression evaluates 1 (true) then loop repeats again otherwise if conditional expression evaluates 0 (false) loop terminates.

The above two steps are repeated until loop condition is met.

Example:

#include <stdio.h>

void main()

{

    /* Loop counter variable declaration */

    int n=1`;

    do

    {

        /* Body of loop */

        printf("%d ", n);

        /* Update loop counter variable */

        N = n + 1;

    } while(n <= 20); /* Loop condition */

}