O-LevelComputer ScienceAlgorithm design and problem-solving2210/22 · May/June 2023 · Q710 Marks

An algorithm has been written in pseudocode to calculate a check digit for a four-digit number. The algorithm then outputs the five-digit number including the check digit. The algorithm stops when -1 is input as the fourth digit. 01 Flag ← FALSE 02 REPEAT 03 Total ← 0 04 FOR Counter ← 1 TO 4 05 OUTPUT "Enter a digit ", Counter 06 INPUT Number[Counter] 07 Total ← Total + Number * Counter 08 IF Number[Counter] = 0 09 THEN 10 Flag ← TRUE 11 ENDIF 12 NEXT Counter 13 IF NOT Flag 14 THEN 15 Number[5] ← MOD(Total, 10) 16 FOR Counter ← 0 TO 5 17 OUTPUT Number[Counter] 18 NEXT 19 ENDIF 20 UNTIL Flag

(a)Give the line number(s) for the statements showing: Totalling Count-controlled loop Post-condition loop[3]
(b)Identify the three errors in the pseudocode and suggest a correction for each error. Error 1 Correction Error 2 Correction Error 3 Correction[3]
(c)The algorithm does not check that each input is a single digit. Identify the place in the algorithm where this check should occur. Write pseudocode for this check. Your pseudocode must make sure that the input is a single digit and checks for -1 Place in algorithm Pseudocode[4]

Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

[3] • 07 • 04/12 or 16/18 • 02/20
[3] One mark for each error identified and correction • Line 07 Total ← Total + Number * Counter should be Total ← Total + Number[Counter] * Counter • Line 08 IF Number[Counter] = 0 should be IF Number[Counter] = -1 // should be IF Number[Counter] < 0 • Line 16 FOR Counter ← 0 TO 5 should be FOR Counter ← 1 TO 5
[4] One mark for place in algorithm (max one) • around lines 05 and 06 • line 07 • (immediately) after the input of the number Three marks pseudocode One mark for each point (max three) • Use of REPEAT ... UNTIL // any working loop structure • check for >0 // >=0 • check for <10 // >9 • check for whole number • check for -1 • check for length of digit <> 1 Example REPEAT OUTPUT "Enter a digit " INPUT Number[Counter] UNTIL Number[Counter] = Round(Number[Counter],0) AND ((Number[Counter] = -1) OR (Number[Counter] > 0 AND Number[Counter] < 10))

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