The counter update clause

(Display Figure 6.)

The counter update clause is executed once during each execution of the body of the loop. 

It is not executed until after the last statement in the body of the loop has been executed.

In other words, it is executed immediately before control transfers back up to the conditional clause to make the next test.

The update clause is used to increment the counter, eventually causing the conditional clause to evaluate to false, causing the loop to terminate.