The use of the three basic programming constructs used to control the flow of a program: Iteration (count- and condition-controlled loops)

Iteration means repeating a set of instructions. There are two main types: count-controlled loops (FOR loops) that repeat a fixed number of times, and condition-controlled loops (WHILE and DO-UNTIL) that repeat until a condition changes. Loops can also be nested, meaning one loop inside another.

Key points

• FOR loops are count-controlled: they repeat a FIXED number of times using a counter variable.
• WHILE loops are condition-controlled: they check a condition BEFORE each iteration and repeat while it is True.
• DO-UNTIL loops check the condition AFTER each iteration, so the code always runs at least once.
• Use a WHILE loop when you don't know in advance how many iterations are needed (e.g. 'repeat until user enters stop').
• Use a FOR loop when you know exactly how many iterations (e.g. 'ask 3 questions', 'check each item in an array').
• Loops can be nested: a loop inside another loop (e.g. sorting algorithms have nested loops).
• Exam Tip:While loops for 'repeat until stop' — the condition must eventually become False. while team != "stop" is common.
• Exam Tip:Make sure inputs that change the loop condition are INSIDE the loop, not just outside it. A typical mistake is to calculate but not re-ask for input.
• Exam Tip:Initialise totals/counters BEFORE the loop, not inside it (otherwise they reset each iteration — a classic logic error).
• Exam Tip:while hours != 0 is safer than while hours > 0 (user could enter -1 and bypass the check).
• Common Mistake:In Python, for x in list: — x is the item VALUE, not an index. Do not then try to access list[x] as x is not an index number.