C Programming: Looping Statements

LOOPING STATEMENTS

 

Introduction

1. The while loop

2. The do..while loop

3. The for loop

4. jump statements

4.1. The break statement

4.2 The continue Statement

4.3 The goto statement

INTRODUCTION

So far, we have discussed, sequential execution of the program. In some  situations there is a need to repeat a set of instructions in specified number of times or until a particular condition is satisfied. This repetitive operations are done through a loop control structure. The loop is defined as the block of statements which are repeatedly executed for certain number of times.

The loop in a program consists two parts, one is body of the loop and another one is control statement. The control statement is used to test the condition and then directs the repeated execution of the statements in the body of the loop. Any looping statement, would include the following steps:

i) Initialization of a condition variable.

ii) Test the control statement.

iii) Executing the body of the loop depending on the condition.

iv) Updating the condition variable.

The following are the loop structures available in 'C'

while..

do...while

for...

 

1. The while loop

It is a repetitive control structure, used to execute the statements within the body, until the condition becomes false. The while loop is an entry controlled loop statement, which means the condition is evaluated first and if it is true, then the body of the loop is executed.

After executing the body of the loop, the condition is once again evaluated and if it is true, the body is executed once again, the process of repeated execution of the body of the loop continues until the condition finally becomes false and the control is transferred out of the loop.

Syntax:

while(condition)

{

   ………….

   body of the loop;

   ………

}

The body of the loop may have one or more statements, the blocking with the braces are needed only if the body contains two or more statements.

Example 1: Addition of numbers up to 10 by using the while loop.

/* Program for addition of numbers using while loop */

#include <stdio.h>

main ()

{

         int i=1,sum=0; /* Local definitions */

         while (i<=10)

         {    /* while loop starting */

                  sum=sum+i;

                  i++;

         } /* while */

         printf("The sum of numbers upto 10 is %d", sum);

} /* main */

OUTPUT

The sum of numbers upto 10 is 55

EXPLANATION: Body of the loop executes until the condition becomes false. When the condition is false the printf() displays the result.

Example 2:  Program to reverse the given number using while loop.

/* Program to reverse the number */

#include<stdio.h>

#include <conio.h>

void main()

{

       int number,digit, rev=0; /* Local definitions */

       clrscr();

       printf("Enter the number : ");

       scanf("%i", &number);

       while(number != 0) /* while loop checks the condition */.

       {

              digit number %10;

              rev=rev*10+ digit;

              number = number/10;

       } /* while */

       printf("%d", rev);

       getch();

} /* main */

OUTPUT 1:

Enter the number : 9293

3929

OUTPUT 2:

Enter the number: 9081

1809

 

2. The do..while loop

The while loop makes a test of condition before the loop is executed. Therefore, the body of the loop may not be executed at all, if the condition is not satisfied at the first attempt. In some situations, it may be necessary to execute the body of the loop, before the test condition is performed. In such a situation the do..while loop is useful.

It is also repetitive control structure and executes the body of the loop once irrespective of the condition, then it checks the condition and continues the execution until the condition becomes false.

Syntax:

do

{

       ………..

       body of the loop;

       …………

} while(condition);

Here, the statements within the body of the loop is executed once, then it evaluates the condition, if it is true, then it executes body until the condition becomes false.

Example 1: Addition of numbers up to 10 by using the do..while loop.

/* Program for addition of numbers using do..while loop */

#include <stdio.h>

main()

{

       int i=1,sum=0; /* Local definition */

       do

       {

              /*do..while loop starting */

              sum=sum+i;

              i++;

       } /* do while **/

       while(i<=10);

       printf("Sum of the numbers up to 10 is ...%d", sum);

} /* main */

OUTPUT

Sum of the numbers up to 10 is ...55

EXPLANATION: The do..while loop is executes the body until i<=10 and calculates the sum and increment the i value by 1.

Example 2: Program to print n numbers using do..while loop.

/* Program to print n numbers */

#include <stdio.h>

#include <conio.h>

void main()

{

       int i,n; /* Local definitions */

       /* Statements */

       printf("Enter the number : ");

       scanf("%d", &n);

       i = 0;

       do

       {

              printf("the numbers are %d\n",i);

              i = i + 1;

       } /* do..while */

       while(i<n);

} /* main */

OUTPUT

Enter the number : 6

the numbers are 0

the numbers are 1

the numbers are 2

the numbers are 3

the numbers are 4

the numbers are 5

EXPLANATION: This program prints n numbers on the screen, where n value can be supplied by the user.

Comparison between while and do while statements.

while

1. This is the top tested loop.

2. The condition is first tested, if the condition is true then the block is executed until the condition becomes false.

3. Loop will not be executed if the condition is false.

do....while

1. This is the bottom tested loop.

2. It executes the body once, after it checks the condition, if it is true the body is executed until the condition becomes false.

3. Loop is executed at least once even though the condition is false.

 

3. The for loop

The for loop is another repetitive control structure, and is used to execute a set of instructions repeatedly, until the condition becomes false.

The assignment, incrementation or decrementation and condition checking is done in for statement only, where as other control structures are not offered all these features in one statement.

Syntax:

for(initialise counter; condition; counter)

{

       ……….

       body of the loop;

      ………

}

for loop has three parts

i) Initialise counter is used to initialize counter variable.

ii) Test condition is used to test the condition.

iii) Increment/decrement counter is used to increment or decrement counter variable.

If there is a single statement within the for loop, the blocking with braces is not necessary, if more than one statement includes in body of the loop, the statements within the body must be blocked with braces.

Example 1: Addition of number upto 10 by using the for loop.

/* Program for addition of numbers using for loop */

#include <stdio.h>

main()

{

       int i,sum=0; /* Local definitions */

       for(i=1;i<=10;i++)

       { /* for loop starting */

              sum=sum+i;

       } /* for */

       printf("The addition of numbers upto 10 is %d", sum);

} /* main */

OUTPUT

The addition of numbers upto 10 is 55.

EXPLANATION: In the program the variables i and sum are declared as an integer type variable. The body of the for loop is executed at 10 times, each time the sum is calculated as sum=sum+i. Finally the prints the value of sum as output.

Note that the initialisation, testing and incrementation of loop is done in the for statement itself. Now, the above program is written in different ways as given below.

main()

{

       int i;

       for(i=1;i<=10;)

       {

              printf("%d\n",i); i=i+1;

       }

}

Here the incrementation is done within the body of the for loop and. not in the for statement. Note that in spite of this, the semicolon after the condition is necessary.

main()

{

       int i;

       for(i=0;i++<10;)

              printf("%d\n",i);

}

Here the comparison as well as the incrementation is done through the same statement, i++<10. Since the ++' operator adds after 'i' then comparison is done followed by incrementation. Note that it is necessary to initialise 'i' to 0.

Example 2: Program to print the numbers in decreasing and increasing order using for loop structure.

#include <stdio.h>

#include <conio.h>

void main()

{

   int n,x,y; /* Local definitions */

   printf("Enter a number: ");

   scanf("%d", &n);

   x=y=n;

   printf("‒‒‒\n");

   printf("(‒‒) (++)\n");

   printf("‒‒‒‒‒‒‒‒‒‒‒\n");

   for(;;(x++,y‒‒))

   {

      printf("%d\t%d\n",y,x);

      if(y==0)

      break;

   } /* for */

   getch();

} /* main */

OUTPUT:

Enter a number: 5

-------------

(‒‒)       (++)

----------

5       11

4       12

3       13

2       14

1       15

0       16

EXPLANATION: The program prints the increment and decrement values of upto given n number using for loop structure.

 

4. JUMP STATEMENTS

There are statements used to control the flow in programs discussed here.

1. The break statement

The break statement is used to terminate the loop. When the keyword break is used inside any 'C' loop, control is automatically transferred to the first statement after the loop. A break is usually associated with an if statement.

When the break statement is encountered inside a loop, the loop is immediately exited and the program continues with the statement immediately following the loop.

Syntax: break;

Example: Program to print the number upto 5 using break statement.

/* Program using break statement */

#include <stdio.h>

#include <conio.h>

main ()

{

     int i;

     for(i=1;i<=10;i++)

     {

          if(i==6)

               break; /* break statement */

          printf("%d", i);

     } /* for */

} /* main */

OUTPUT:

1 2 3 4 5

EXPLANATION: printf value of 'ï' upto 5 when ï' reaches 6 the if statement becomes true. So the break statement transfers the control to the outside of the for loop.

Terminating loop with break statement

2. The continue Statement

In some situations, we want to take the control to the beginning of the loop, bypassing the statements inside the loop which have not yet been executed, for this purpose the continue is used. When the statement continue is encountered inside any 'C' loop control automatically passes to the beginning of the loop.

Syntax: continue;

Like break statement, the continue statements also associate with if statement.

Example: Calculate the sum of the given positive numbers.

/* Program to calculate the sum of the given positive numbers */

#include <stdio.h>

main

{

     int i,n,sum=0; /* Local definitions */

     /* Statements */

     for(i=1;i<=5;i++)

     {

          printf("Enter any number.....\n");

          scanf("%d", &n);

          if(n<0)

               continue;

          else

               sum sum+n;

     }/* for */

     printf("Sum is….%d", sum);

} /* main */

OUTPUT

Enter any number.....10

Enter any number.....5

Enter any number.....15

Enter any number.....25

Enter any number.....‒10

Enter any number.....50

Sum is .....105.

EXPLANATION : Here, the for loops reads 5 numbers, for each number it checks whether it is less than zero, if it is less than zero it can not add and asks for next number (i.e. the control will automatically transfer to the for loop using the continue), if it is greater than zero, it adds to the variable sum, finally after completing the loop it displays the results.

Continue loop with continue statement

Difference between the break and continue

break

1. Break statement takes the control to the outside of the loop.

2. It is also used in switch statement.

3. Always associated with if condition in loops.

continue

1. Continue statement takes the control to the beginning of the loop.

2. This can be used only in loop statements.

3. This is also associated with if condition.

3. The goto statement

So far, we have seen a number of ways of controlling the flow of execution based on the conditions. 'C' provides the goto statement to transfer control unconditionally from one place to another place in the program.

A goto statement can cause program control almost anywhere in the program unconditionally.

The goto statement requires a label to identify the place to move the execution. A label is a valid variable name and must be ended with colon (:).

Syntax :