Iteration: WHILE and REPEAT

WHILE tests before each pass

• A WHILE loop checks its test before each pass.
• If the test is false at the start, the body runs zero times.
• The block ends at ENDWHILE.

Count ← 1
WHILE Count <= 3 DO
    OUTPUT Count        // 1, 2, 3
    Count ← Count + 1
ENDWHILE

REPEAT tests after each pass

• A REPEAT loop checks its test after each pass.
• So the body always runs at least once.
• It ends at UNTIL.

N ← 0
REPEAT
    N ← N + 2
    OUTPUT N            // 2, 4, 6
UNTIL N >= 6

Watch out

• Something inside the loop must move the test toward stopping, or it runs forever.
• WHILE may run 0 times; REPEAT always runs at least 1 time.

Common mistakes

• WHILE tests the condition first; REPEAT ... UNTIL tests after, so it runs at least once.
• Change something inside, or the loop never ends.

Now you try

• Pick WHILE or REPEAT for each task.
• Press Check answer to test it.