Examples of indefinite (condition controlled) iteration with the condition…

The idea

Indefinite iteration repeats code an unknown number of times, continuing while or until a condition is met. When the condition is tested at the end, the loop body always runs at least once before the test, so it is a post-condition (post-test) loop. In AQA pseudocode this is written as REPEAT, then the instructions, then UNTIL followed by the condition. The loop stops when the condition becomes true, so the body repeats while the condition is false. Contrast this with a condition at the start (WHILE), which may run zero times. A typical example is repeatedly asking for a password until the correct one is entered: the prompt must appear at least once. Recognising where the condition sits tells you the minimum number of executions.

What earns the marks

• Identifies that indefinite (condition controlled) iteration repeats an unknown number of times until a condition is satisfied.
• Recognises that placing the condition at the end makes it a post-condition loop whose body executes at least once.
• Matches the REPEAT … UNTIL structure to the condition-at-the-end form.
• Distinguishes it from a condition-at-the-start WHILE loop, which may execute zero times.
• Explains that the loop continues while the condition is false and stops when it becomes true.

Common mistakes and how to put them right

• Assuming a condition-at-the-end loop can run zero times.

  Put it right: The body always runs at least once before the test.

• Confusing REPEAT … UNTIL with a WHILE loop.

  Put it right: In REPEAT … UNTIL the condition is checked after the body.

• Thinking the loop stops when the condition is false.

  Put it right: It repeats until the condition is true.

Exam tips

• Look at where the condition is written: at the end means post-condition, at the start means pre-condition.
• State the minimum number of executions to justify your choice.
• Use a concrete example such as entering a password until it is correct to show the body must run at least once.