A theoretical understanding of condition(s) at either end of an iterative…

The idea

You must understand, in theory, how a condition placed at the start or the end of an iterative structure behaves, even if the programming language you use does not provide both forms. A start-tested loop checks its condition before each pass, so the body may never run. An end-tested loop runs the body first and checks afterwards, so the body always runs at least once. This is conceptual knowledge about control flow, not a demand that your language supports every loop type.

For example: If your language only offers a while loop, you can still describe and reason about an end-tested loop and predict its behaviour, and you can emulate one using a while loop with a flag.

What earns the marks

• The condition at the start of a loop is evaluated before the body, so the body may execute zero times.
• The condition at the end of a loop is evaluated after the body, so the body executes at least once.
• Theoretical understanding is required independently of whether a particular language supports both loop forms.
• Predicting loop behaviour: given initial values, state how many times the body runs for start-tested versus end-tested conditions.
• Emulating an end-tested loop in a language that lacks one, for example using a while loop with a Boolean flag.

Common mistakes and how to put them right

• Thinking the two forms behave identically.

  Put it right: Is that start-tested loops may run zero times while end-tested loops run at least once.

• Assuming an end-tested loop checks its condition before the first pass.

  Put it right: Is that the body runs first and the condition is checked afterwards.

Exam tips

• State clearly whether the condition is checked before or after the body when comparing loop forms.
• Use a trace table with initial values to show how many times each loop body runs.
• When a language lacks a loop form, describe how it could be emulated rather than claiming it is impossible.