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 Counter : INTEGER
Counter ← 3
WHILE Counter > 0
  OUTPUT Counter
  Counter ← Counter - 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 Counter : INTEGER
Counter ← 0
REPEAT
  Counter ← Counter + 1
  OUTPUT Counter
UNTIL Counter = 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

Trace a starting value of zero as well as a positive value. Decide whether any output should happen before the first condition test. Do not combine the opening of one loop type with the ending of another.

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

Now you try

• Write one WHILE and one REPEAT.

Independent transfer

The final task has no supplied algorithm. Plan its inputs, processing and outputs, then solve it before opening support. Check more than the sample case.