What is the primary difference between a count-controlled loop and a condition-controlled loop?

A count-controlled loop repeats a specific number of times known in advance, while a condition-controlled loop repeats until a specific logical state is reached, which may not be known beforehand.

How do WHILE loops and REPEAT...UNTIL loops differ in their minimum number of executions?

A WHILE loop has a minimum of zero executions because the condition is checked before entry. A REPEAT...UNTIL loop has a minimum of one execution because the condition is checked after the body runs.

When should a programmer choose a WHILE loop over a REPEAT...UNTIL loop?

A WHILE loop should be chosen when there is a possibility that the loop body should not execute at all, such as when processing a list that might be empty.

What is an 'off-by-one' error in the context of iteration?

An off-by-one error occurs when a loop iterates one time too many or one time too few, often caused by using the wrong relational operator (e.g., using < instead of <=) or incorrect start/end values.

What causes an infinite loop and how can it be avoided?

An infinite loop occurs when the loop's termination condition is never met. It is avoided by ensuring the loop body contains logic that updates the control variable or condition toward the exit state.

What is the danger of using a fixed increment in a condition-controlled loop?

If the increment does not lead the variable to eventually satisfy the exit condition (e.g., incrementing away from the target), the loop will become infinite and the program will hang.

Define 'Iteration' in the context of computer science.

Iteration is a control structure that allows a sequence of instructions to be executed repeatedly based on a count or a logical condition.

What is a 'Step' or 'Increment' in a FOR loop?

The step is the value by which the loop counter variable is increased or decreased after each iteration of the loop body.

What is a 'Nested Loop'?

A nested loop is an iteration structure placed inside the body of another loop, where the inner loop completes all its iterations for every single iteration of the outer loop.

In pseudocode, what does the 'UNTIL' keyword signify in a loop?

The 'UNTIL' keyword specifies the condition that must become True for the loop to stop executing; the loop continues as long as the condition remains False.

Iteration | Cambridge International Examinations AS-Level Computer Science

Q: What causes an infinite loop and how can it be avoided?

An infinite loop occurs when the loop's termination condition is never met. It is avoided by ensuring the loop body contains logic that updates the control variable or condition toward the exit state.

Q: What is the danger of using a fixed increment in a condition-controlled loop?

If the increment does not lead the variable to eventually satisfy the exit condition (e.g., incrementing away from the target), the loop will become infinite and the program will hang.

Q: Define 'Iteration' in the context of computer science.

Iteration is a control structure that allows a sequence of instructions to be executed repeatedly based on a count or a logical condition.

Q: What is a 'Step' or 'Increment' in a FOR loop?

The step is the value by which the loop counter variable is increased or decreased after each iteration of the loop body.

Q: What is a 'Nested Loop'?

A nested loop is an iteration structure placed inside the body of another loop, where the inner loop completes all its iterations for every single iteration of the outer loop.

Q: In pseudocode, what does the 'UNTIL' keyword signify in a loop?

The 'UNTIL' keyword specifies the condition that must become True for the loop to stop executing; the loop continues as long as the condition remains False.

Revision Notes

AS-Level

Cambridge International Examinations

Computer Science

11. Programming

Iteration

Summary

Iteration is a fundamental programming construct that enables the repeated execution of a block of code. By using loops, developers can process large datasets, perform repetitive calculations, and manage program flow efficiently without manually duplicating instructions.

1. Definition & Core Concepts

Iteration is the process of repeating a specific set of instructions or a block of code multiple times until a specific goal is reached.
A Loop is the structural implementation of iteration within a program, consisting of a loop body (the code to repeat) and a control mechanism.
The Loop Control Variable is a variable used to track the progress of the iteration, such as a counter in a count-controlled loop or a flag in a condition-controlled loop.

Flowchart showing the basic logic of an iteration loop where a condition is checked to determine if the body should execute or the loop should terminate.

2. Count-Controlled Iteration (FOR Loops)

Count-controlled loops are used when the number of repetitions is known before the loop begins execution.
The structure typically involves a start value, an end value, and an optional step value (increment or decrement).
In pseudocode, this is represented as FOR <identifier> ← <start> TO <end> STEP <increment>, where the identifier is automatically updated after each pass.
These loops are ideal for iterating through data structures with a fixed size, such as arrays or lists.

3. Condition-Controlled Iteration

Pre-Condition Loops (WHILE)

A WHILE loop checks the condition before the loop body is executed; if the condition is initially false, the body may never run.
This is used when the number of iterations is unknown and depends on dynamic factors like user input or sensor data.

Post-Condition Loops (REPEAT...UNTIL)

A REPEAT...UNTIL loop executes the loop body first and then checks the condition at the end.
Because the check happens at the exit point, the loop body is guaranteed to execute at least once.
Note that WHILE loops usually continue while a condition is true, whereas REPEAT loops continue until a condition becomes true.

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions