Conditional Loops

Conditional Loops

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 pseudocode syntax:

WHILE condition DO
    <loop body>
END WHILE

Python syntax:

while condition:
    # loop body (indented by 4 spaces)

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 pseudocode syntax:

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

For most N5 exam questions you will write REPEAT…UNTIL in pseudocode. In Python, you will typically use a WHILE loop with the input collected before the loop and again inside it.

Input validation — the most important pattern

Input validation is the most commonly examined use of conditional loops. The standard pattern uses a WHILE loop to keep asking the user for input until they provide a value within the acceptable range:

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)

Notice that the input is collected twice: once before the loop (so the condition can be checked immediately) and once inside the loop (so a new value is read on each iteration). If the input is only inside the loop, the condition has no value to check at the start.

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