A weather station technician enters a sequence of temperature readings, in degrees Celsius, one at a time. The value -999 is used as a sentinel value to signal that there is no more data, and is never itself a real temperature reading. This pseudocode algorithm totals the real r…

Answer: ((a)) This is a pre-condition loop, because the condition on line 07 (Reading <> -999) is tested before the loop body on lines 08 to 10 runs for the first time. If the very first value read on line 06 were already -999, the body would never execute at all. ((b)) Reading = 18: Total = 18, Count = 1. Reading = 22: Total = 40, Count = 2. Reading = -5: Total = 35, Count = 3. Reading = 31: Total = 66, Count = 4. Reading = -999: condition fails, loop stops. ((c)) Total = 0 and Count = 0. The priming read on line 06 sets Reading to -999 immediately, so when line 07 is tested for the first time, -999 <> -999 is false and the loop body never runs, leaving Total and Count at their initial values.. Part (a): Pre-condition or post-condition? A WHILE ... DO ... ENDWHILE loop is always a pre-condition loop: the condition is written next to the WHILE keyword itself and is checked before the statements inside the loop are ever executed. I…

IGCSE Computer Science 0478: Tracing a WHILE Loop with a Sentinel Value

01 DECLARE Reading : INTEGER 02 DECLARE Total : INTEGER 03 DECLARE Count : INTEGER 04 Total ← 0 05 Count ← 0 06 INPUT Reading 07 WHILE Reading <> -999 DO 08 Total ← Total + Reading 09 Count ← Count + 1 10 INPUT Reading 11 ENDWHILE 12 OUTPUT Total 13 OUTPUT Count

Part (a): Pre-condition or post-condition?

A WHILE ... DO ... ENDWHILE loop is always a pre-condition loop: the condition is written next to the WHILE keyword itself and is checked before the statements inside the loop are ever executed.

In this algorithm, the condition Reading <> -999 is on line 07, and it is tested before line 08 (Total ← Total + Reading) runs for the very first time. This is different from a REPEAT ... UNTIL loop, where the condition is only checked after the body has already run once. A REPEAT loop is guaranteed to execute its body at least once, but this WHILE loop is not. [2 marks]: [1] for correctly identifying “pre-condition”, [1] for a correct explanation referencing line 07 (or an equivalent explanation of the condition being checked before the body).

Part (b): Tracing the WHILE loop

Total and Count both start at 0 (lines 04–05), and Reading is first given a value by the priming read on line 06, before the loop is even reached.

Pass	Reading (start of pass)	Reading <> -999 ?	Total	Count	Reading (re-read at end of pass)
,	18 (from line 06)	,	0	0	-
1	18	true	18	1	22
2	22	true	40	2	-5
3	-5	true	35	3	31
4	31	true	66	4	-999
,	-999	false → loop stops	66	4	,

[4 marks]: [1] for correctly tracing Total to a final value of 66, [1] for correctly tracing Count to a final value of 4, [1] for correctly excluding the sentinel value -999 from both Total and Count, [1] for a fully correct trace table showing all four genuine passes.

Part (c): The edge case where the first reading is the sentinel

Line 06 performs the priming read, so Reading is set to -999 immediately, before the WHILE condition on line 07 is tested for the first time. The test Reading <> -999 becomes -999 <> -999, which is false. Because this is a pre-condition loop, the loop body (lines 08–10) is skipped entirely, it never runs, not even once.

Total and Count therefore keep the initial values assigned on lines 04 and 05:

Total = 0
Count = 0

[2 marks]: [1] for recognising that the loop body does not execute at all, [1] for the correct final values Total = 0 and Count = 0.

Final answers

(a) Pre-condition loop. The condition on line 07 is tested before line 08 first runs.
(b) Trace as shown above. Total rises 18, 40, 35, 66; Count rises 1, 2, 3, 4.
(c) Total = 0, Count = 0 (the loop body never executes).