Loop statements

Definite iteration is where the set of actions is performed a known number of times. The number might be determined by the program specification, or it might not be known until the program is executing, just before starting the iteration.

Ada provides the FOR statement for definite iteration.

Indefinite iteration is where the set of actions is performed a unknown number of times. The number is determined during execution of the loop.

Ada provides the WHILE statement and general LOOP statement for indefinite iteration.

Loop Statements Compared

WHILE vs FOR

The while loop is most general

• used for indefinite iteration
• by setting value of variables used in the test, can be made to execute at least once, or maybe never

For loops:

• used for definite iteration (know number of times to run before loop)
• can be replaced by a while loop, but usually not as elegant

WHILE vs General LOOP

While:

• obvious where loop exits
• can guarantee that the entry test has been passed when executing the statements of the loop body

General loop:

• sometimes code is simpler
• there is no guarantee when executing the statements before the test (this may be the first time through the loop)

Which loop statement to use?

1. For definite loops, use for
   • automated control
   • tells reader that number of iterations known
2. For indefinite loops, where pre-test is natural, use while
   • simpler reasoning
3. For other indefinite loops, use general loop
   • less cluttered code

Usually prefer for and while.