Programming concepts: DO … Instructions here … WHILE NotSolved

The idea

A post-condition (post-test) loop runs its body first and only then tests the condition, so the instructions always execute at least once. In the pattern DO … Instructions here … WHILE NotSolved, the program performs the instructions, then evaluates NotSolved; if it is true the loop repeats, and if it is false the loop ends. This differs from a pre-condition loop such as WHILE NotSolved … ENDWHILE, which tests first and may skip the body entirely. A typical use is repeatedly asking for input until a valid value is entered, or repeating a game turn until the puzzle is solved. When answering, trace the order of execution: body, then test, then either repeat or exit.

What earns the marks

• Identifies the structure as a post-condition (post-test) loop because the condition is checked after the body runs.
• States that the instructions execute at least once, even if the condition is false on the first test.
• Explains that the loop repeats while NotSolved remains true and terminates when NotSolved becomes false.
• Contrasts it with a pre-condition WHILE loop, which tests before the body and may run zero times.
• Applies the pattern to a realistic case, such as repeating input until a valid value is entered.

Common mistakes and how to put them right

• Treating the loop as testing before the body runs.

  Put it right by noting the condition appears after the instructions, so the body always runs at least once.

• Assuming the loop stops when NotSolved is true.

  Put it right by stating it continues while NotSolved is true and stops when it is false.

• Confusing this with a FOR loop that repeats a fixed number of times.

  Put it right by explaining the repetition depends on a condition, not a counter.

Exam tips

• Underline where the condition sits in the code before deciding whether it is pre-test or post-test.
• Trace a short example with two or three passes, writing the value of NotSolved after each test.
• When comparing loop types, state clearly which one guarantees at least one execution.