Iteration: WHILE and REPEAT

WHILE tests first

• A pre-condition loop tests the condition, then maybe runs the body.
• If the condition is already FALSE, the body never runs.
• The block ends at ENDWHILE.

DECLARE N : INTEGER
N ← 3
WHILE N > 0
  OUTPUT N
  N ← N - 1
ENDWHILE

REPEAT runs, then tests

• A post-condition loop runs the body, then tests.
• The body runs at least once.
• The loop stops when the condition becomes TRUE.

DECLARE N : INTEGER
N ← 0
REPEAT
  N ← N + 1
  OUTPUT N
UNTIL N = 3

Watch out

• WHILE stops on FALSE. REPEAT stops on TRUE. They are opposite tests.
• Something inside a WHILE must change the condition, or the loop never ends.

Common mistakes

• ENDWHILE closes a WHILE. UNTIL closes a REPEAT.
• A REPEAT that should stop at 3 must update N before the test.

Now you try

• Write one WHILE and one REPEAT.