Repetition — the WHILE Loop

By the end you should be able to: Write pseudocode using a WHILE loop, identify the three parts every condition-controlled loop needs, and recognise causes of infinite loops.

A WHILE loop repeats as long as a condition remains true. Use it when the number of iterations is not known before the loop starts.

The form

WHILE condition DO
    statements
ENDWHILE

The condition is tested before each iteration. If it is false at the very first test, the body never runs at all — a pre-test or entry-controlled loop.

i ← 1
WHILE i <= 5 DO
    PRINT i
    i ← i + 1
ENDWHILE

The three parts every WHILE needs

Miss any one and the loop misbehaves. Check all three, every time.

Part	Where	Purpose
Initialisation	Before the loop	Give the control variable a starting value
Condition	In the WHILE	Decide whether to continue
Update	Inside the body	Move the variable towards making the condition false

i ← 1                    1. initialise
WHILE i <= 5 DO          2. condition
    PRINT i
    i ← i + 1            3. update  ← the one people forget
ENDWHILE

Omit the update and i stays 1 forever. The condition never becomes false. Infinite loop.

Causes of infinite loops

1. No update. As above.

2. Update in the wrong direction.

i ← 1
WHILE i <= 5 DO
    i ← i - 1            moves away from the condition
ENDWHILE

3. A condition that can never be false.

WHILE x <> 5 DO          if x is a float, exact equality may never hold

Prefer < or > over = and <> when the variable is a float, for exactly the floating-point reasons met in Module 1.

4. The update is inside an IF that doesn't always run.

WHILE count < 10 DO
    READ x
    IF x > 0 THEN
        count ← count + 1
    ENDIF
ENDWHILE

If the user only ever enters negative numbers, count never advances. Sometimes this is intended; usually it isn't. Ask yourself whether every path through the body makes progress.

Sentinel-controlled loops

A sentinel is a special value marking the end of input.

BEGIN
    sum ← 0
    READ x
    WHILE x <> 0 DO              0 is the sentinel
        sum ← sum + x
        READ x
    ENDWHILE
    PRINT "Sum is", sum
END

Note the structure: read once before the loop, then read again at the end of the body. This is the priming read, and it is what students most often get wrong.

Without the priming read, the first WHILE test examines an uninitialised variable. With the read only at the top of the body, the sentinel itself gets added to the sum before the condition is retested.

The sentinel must be a value that cannot occur as real data. Using 0 to end a list of marks is fine; using it to end a list of temperatures is not.