Programming concepts: Use nested selection and nested iteration structures.

The idea

Nesting means placing one control structure inside another. Nested selection puts an IF (or IF…ELSE) inside another IF, so a second decision is only reached when the outer condition is met; for example, checking a score is above 50, then inside that checking whether it is above 80 to award a higher grade. Nested iteration places one loop inside another, such as an outer loop over rows and an inner loop over columns to fill a grid, where the inner loop completes fully for each single pass of the outer loop. Correct indentation shows which statements belong to which block. When answering, identify the outer and inner structures, state how many times the inner block runs, and trace the order of execution.

What earns the marks

• Defines nesting as placing one control structure inside another, such as an IF inside an IF or a loop inside a loop.
• Explains that in nested selection the inner decision is only evaluated when the outer condition is true.
• Explains that in nested iteration the inner loop completes all its repetitions for each single pass of the outer loop.
• Traces execution order correctly, for example an outer loop of 3 passes with an inner loop of 4 giving 12 inner executions.
• Recognises that indentation shows which statements belong to the inner block.

Common mistakes and how to put them right

• Thinking the inner loop runs only once overall.

  Put it right by stating it restarts and completes for every pass of the outer loop.

• Believing both branches of nested selection always run.

  Put it right by explaining the inner decision is reached only if the outer condition is satisfied.

• Ignoring indentation and misreading which statements are inside which block.

  Put it right by matching each statement to its enclosing structure.

Exam tips

• Label the outer and inner structures before tracing any values.
• For nested loops, multiply the repetition counts to find total inner executions, then check with a short trace.
• Use indentation in any code you write so the nesting is unambiguous to the reader.