The break Statement: C programing The Loop Control Structure

We often come across situations where we want to jump out of a loop instantly, without waiting to get back to the conditional test. The keyword break  allows us to do this. When break  is encountered inside any loop, control automatically passes to the first statement after the loop. A break is usually associated with an
if. As an example, let’s consider the following example.

Example:  Write a program to determine whether a number is prime or not. A prime number is one, which is divisible only by 1 or itself.

All we have to do to test whether a number is prime or not, is to divide it successively by all numbers from 2 to one less than itself. If remainder of any of these divisions is zero, the number is not a prime. If no division yields a zero then the number is a prime number. Following program implements this logic

main( )
{
      int   num, i ;

      printf ( "Enter a number " ) ;
      scanf ( "%d", &num ) ;

      i = 2 ;
      while ( i <= num - 1 )
 {
            if ( num % i == 0 )
  {
                  printf ( "Not a prime number" ) ;
   break ;
  }
  i++ ;
 }
if ( i == num )
            printf ( "Prime number" ) ;
}

In this program the moment num % i turns out to be zero, (i.e. num is exactly divisible by i) the message “Not a prime number” is printed and the control breaks out of the while loop. Why does the program require the if  statement after the while  loop at all? Well, there are two ways the control could have reached outside
the while loop:

(a)  It jumped out because the number proved to be not a prime.
(b)  The loop came to an end because the value of i became equal to num.

When the loop terminates in the second case, it means that there was no number between 2 to num - 1 that could exactly divide num. That is, num is indeed a prime. If this is true, the program should print out the message “Prime number”. The keyword break, breaks the control only from the while  in which it is placed. Consider the following program, which illustrates this fact.

main( )
{
      int  i = 1 , j = 1 ;

      while ( i++ <= 100 )
 {
            while ( j++ <= 200 )
  {
                  if ( j == 150 )
    break ;
   else
                        printf ( "%d %d\n", i, j ) ;
  }

 }
}

In this program when j equals 150, break takes the control outside the inner while only, since it is placed inside the inner while.