This topic summary covers Knowledge Organiser: Iteration within Iteration (Loops) for GCSE Computer Science. Revise Iteration (Loops) in 3.2 Programming for GCSE Computer Science with 15 exam-style questions and 8 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 8 of 8 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Knowledge Organiser: Iteration
Key Terms
- Iteration: Repeating a block of code — also called a loop
- FOR loop: A count-controlled loop that repeats a fixed number of times
- WHILE loop: A condition-controlled loop that repeats while a condition is true (may run 0 times)
- DO-WHILE loop: A loop that always runs at least once, then checks the condition
- Infinite loop: A loop whose condition never becomes false — a common bug
- Off-by-one error: A bug caused by using the wrong loop boundary (e.g. 1 to 10 vs 0 to 9)
Must-Know Facts
- FOR loops are used when the number of iterations is known in advance
- WHILE loops are used when the number of iterations is unknown
- A WHILE loop may execute zero times if the condition is false at the start
- DO-WHILE always executes the body at least once before checking the condition
- WHILE loop conditions must eventually become false to avoid an infinite loop
Key Concepts
- FOR loop:
for i = 1 to 5 ... next i - WHILE loop:
while condition ... endwhile - DO-WHILE:
do ... until condition - Input validation pattern:
do ... age = input(...) ... until age >= 0 AND age <= 120 - Sum 1 to 100:
total = 0 ... for i = 1 to 100 ... total = total + i ... next i
Common Mistakes
- Using a FOR loop when the number of repetitions is unknown: FOR loops are for a fixed number of iterations — use WHILE or DO-WHILE when repeating until a condition is met (e.g. user input validation)
- Confusing WHILE and DO-WHILE: A WHILE loop may run zero times if the condition is false from the start; a DO-WHILE always runs at least once — this matters when validating user input
- Creating infinite loops by never updating the condition variable: If the variable controlling a WHILE loop never changes inside the loop, the condition never becomes false
- Off-by-one errors in FOR loops:
for i = 1 to 5runs 5 times (1,2,3,4,5);for i = 0 to 4also runs 5 times — confusing the two is a very common bug - Not indenting loop bodies in pseudocode: Indentation is required to show which statements are inside the loop — examiners expect it in written pseudocode answers
Practice questions for Iteration (Loops)
Which type of loop is best used when you know exactly how many times the loop should repeat?
Explain what an infinite loop is, state one cause of an infinite loop, and describe one consequence of running an infinite loop in a program.
Quick recall flashcards
When use a FOR loop?
When you know how many times to repeat
What is iteration?
Repeating a set of instructions