Do Be Do Be Do

C also provides a post-test structure. It looks like:

do {
} while( condition ) ;

For example, a code fragment to print the numbers 0 to 9 using the do-while loop would look like:

i = 0 ;
do {
   printf( "%d\n", i ) ;
   i++ ;
} while( i < 10 ) ;

Now, this does the same thing as the example in the previous part, and it doesn't really matter which one we use since 10 is a constant and won't change. On the other hand, what if instead of 10 we had j (another variable). Now the two versions will operate a little differently. In particular, what happens if j is 0, namely we are asking it to count up from 0 to -1. It shouldn't print anything in that case and doesn't in the pre-test loop. But since the body of the post-test loop is always executed at least once, this version will print 0. It turns out that most of the loops we create are like this. They might not ever be executed depending on the inputs. So we usually find the while statement used more often than the do-while statement.

There is on common looping need that the do-while is best for. Suppose that the user is to input a number and that it must be between 0 and 100 inclusive. We can do this with something like:

do {
   printf( "Enter a number: " ) ;
   scanf( "%d", &x ) ;
} while( x < 0 || x > 100 ) ;

In general, if the condition on which looping depends is initially set inside the loop, then we'll need to execute the loop at least once and we'll use a post-test loop.

What does y equal after this code fragment is executed?
y = 1 ;
x = 5 ;
do {
   y = x * y ;
} while( y < 2 * x ) ;