Nesting of Loops

The way if statements can be nested, similarly whiles and fors can also be nested. To understand how nested loops work, look at the program given below:

/* Demonstration of nested loops */
main( )
{
      int   r, c, sum ;
      for ( r = 1 ; r <= 3 ; r++ )  /* outer loop */
 {
            for ( c = 1 ; c <= 2 ; c++ )  /* inner loop */
  {
                  sum = r + c ;
                  printf ( "r = %d c = %d sum = %d\n", r, c, sum ) ;
  }
 }
}

When you run this program you will get the following output:

r = 1 c = 1 sum = 2
r = 1 c = 2 sum = 3
r = 2 c = 1 sum = 3
r = 2 c = 2 sum = 4
r = 3 c = 1 sum = 4
r = 3 c = 2 sum = 5

Here, for each value of r the inner loop is cycled through twice, with the variable c taking values from 1 to 2. The inner loop

terminates when the value of c exceeds 2, and the outer loop terminates when the value of r exceeds 3. 
As you can see, the body of the outer for loop is indented, and the body of the inner for  loop is further indented. These multiple
indentations make the program easier to understand.  Instead of using two statements, one to calculate sum and another to print it out, we can compact this into one single statement by saying:

printf ( "r = %d c = %d sum = %d\n", r, c, r + c ) ;

The way for  loops have been nested here, similarly, two while loops can also be nested. Not only this, a for loop can occur within a while loop, or a while within a for.