Conditional Loops

Conditional Loops

SQA uses reference language in question papers. You need to be able to read it. You will never be asked to write it. Write Python for implementation questions; design questions use structured-English pseudocode.

What is a conditional loop?

A conditional loop (also called a condition-controlled loop) repeats a block of code while a condition is true — or until a condition becomes true. The defining feature is that you do not know in advance how many times the loop will run. The number of iterations is determined by what happens during the program's execution, not by a fixed number set before the loop starts.

Conditional loops are the right choice when:

• You want to keep asking the user for input until they enter a valid value (input validation)
• You want to keep processing data until a special "stop" value (sentinel) is entered
• You want to repeat an action until some condition is met

If you already know exactly how many times the loop should run — for example, always 5 scores or always 10 table entries — a fixed loop (SDD9) is the correct choice instead.

The WHILE loop

The WHILE loop checks its condition before each iteration. If the condition is false the very first time it is checked, the loop body never runs at all.

SQA reference language (read only)

Read this syntax pattern; it is not a writing template.

WHILE condition DO
    <loop body>
END WHILE

Python syntax:

while condition:
    pass  # replace with the indented loop body

The condition is a boolean expression. The loop continues repeating as long as the condition evaluates to True. As soon as it evaluates to False, the loop ends and execution continues with the first line after END WHILE (or after the Python loop's indented block).

Key point: the loop body must contain code that can eventually make the condition false. If not, the loop runs forever — this is called an infinite loop.

The REPEAT…UNTIL loop

The REPEAT…UNTIL loop checks its condition after each iteration. This means the loop body always runs at least once, regardless of the condition. The loop continues until the condition becomes true — it repeats while the condition is false.

SQA reference language (read only)

Read this syntax pattern; it is not a writing template.

REPEAT
    <loop body>
UNTIL condition

This is different from a WHILE loop in a subtle but important way: WHILE continues while the condition is true; REPEAT…UNTIL continues until the condition becomes true (i.e., while it is false).

Python equivalent: Python has no built-in REPEAT…UNTIL construct. The closest pattern uses while True with a break:

while True:
    # loop body (always runs at least once)
    if condition:
        break

Read REPEAT…UNTIL in question papers. In Python, either use a while loop that tests the input collected before it, or use while True with a break after the body. Choose a pattern whose logic matches the task.

Input validation — the most important pattern

Input validation is one use of conditional loops. Read this reference-language example: it asks again while the value is outside the acceptable range. Then compare the Python implementation below.

SQA reference language (read only)

RECEIVE score FROM (INTEGER) KEYBOARD
WHILE score < 0 OR score > 100 DO
    SEND "Invalid — enter a value between 0 and 100" TO DISPLAY
    RECEIVE score FROM (INTEGER) KEYBOARD
END WHILE

In Python:

score = int(input("Enter score (0–100): "))
while score < 0 or score > 100:
    print("Invalid — enter a value between 0 and 100.")
    score = int(input("Enter score (0–100): "))
print("Valid score:", score)

In this pattern, one input statement is before the loop and another is inside it. The inner statement runs whenever an invalid value is rejected. The condition needs an assigned value before its first check; a different valid pattern can initialise a variable before the loop or use while True with a break.

Sentinel values

A sentinel value is a special value that signals the end of input — for example, entering −1 to stop adding numbers. The loop checks for the sentinel after each input and stops when it is detected:

total = 0
number = int(input("Enter a number (–1 to stop): "))
while number != -1:
    total = total + number
    number = int(input("Enter a number (–1 to stop): "))
print("Total:", total)

The sentinel value itself (−1) is never added to the total — the condition is checked before the addition.

Comparing fixed and conditional loops