Repetition — the REPEAT-UNTIL Loop

By the end you should be able to: Write pseudocode using REPEAT-UNTIL, distinguish pre-test from post-test loops, and choose the appropriate loop for a given problem.

REPEAT-UNTIL runs the body first and tests afterwards. It is the third loop variant, and the syllabus asks you to solve the sum-of-n-numbers problem with all three.

The form

REPEAT
    statements
UNTIL condition

The body executes, then the condition is tested. If false, repeat. If true, stop.

Two differences from WHILE, both important:

The body always runs at least once. The test comes after.
The condition is a stopping condition, not a continuing one. WHILE continues while true; REPEAT-UNTIL stops when true.

That second point reverses the logic, and reversing it incorrectly is the standard exam error.

WHILE and REPEAT-UNTIL compared

	WHILE	REPEAT-UNTIL
Test position	Before the body	After the body
Minimum iterations	0	1
Condition means	Continue while true	Stop when true
Also called	Pre-test, entry-controlled	Post-test, exit-controlled

The same loop written both ways:

i ← 1                          i ← 1
WHILE i <= 5 DO                REPEAT
    PRINT i                        PRINT i
    i ← i + 1                      i ← i + 1
ENDWHILE                       UNTIL i > 5

Note the conditions are logical opposites: i <= 5 against i > 5. To convert between the forms, negate the condition.

When the difference actually matters

For n = 0 in a sum-of-n-numbers program:

WHILE: count <= 0 is false immediately. Zero iterations. Sum is 0. ✓
REPEAT-UNTIL: the body runs once regardless, reading a number that shouldn't be read. Sum is wrong. ✗

So REPEAT-UNTIL is wrong for that problem unless you guard it:

IF n > 0 THEN
    REPEAT
        ...
    UNTIL count > n
ENDIF

This is the general rule: if zero iterations must be possible, use WHILE.

Where REPEAT-UNTIL is the natural choice

Menu-driven programs and input validation — anywhere the body must run at least once by the nature of the problem.

REPEAT
    PRINT "Enter a mark between 0 and 100"
    READ marks
UNTIL marks >= 0 AND marks <= 100

You must ask at least once, so a post-test loop expresses the intent exactly. A WHILE version needs a priming read and a duplicated prompt:

PRINT "Enter a mark between 0 and 100"
READ marks
WHILE marks < 0 OR marks > 100 DO
    PRINT "Enter a mark between 0 and 100"      duplicated
    READ marks
ENDWHILE

Note the WHILE condition is the negation of the UNTIL condition, by De Morgan:

$\text{NOT}(m \geq 0 \text{ AND } m \leq 100) \equiv (m < 0 \text{ OR } m > 100)$

Getting this negation wrong — writing AND where OR belongs — is a frequent and costly slip.

A note on Python

Python has no repeat-until. The idiom is:

while True:
    marks = int(input("Enter a mark: "))
    if 0 <= marks <= 100:
        break

You still need the concept for pseudocode and for the exam; you just won't write it in Python directly.