O-LevelComputer ScienceAlgorithm design and problem-solving2210/22 · Oct/Nov 2021 · Q410 Marks
The pseudocode algorithm should work as a calculator and output the result. 1 Continue ← 1 2 WHILE Continue = 0 3 OUTPUT "Enter 1 for +, 2 for -, 3 for * or 4 for /" 4 INPUT Operator 5 OUTPUT "Enter the first value" 6 INPUT Value1 7 OUTPUT "Enter the second value" 8 OUTPUT Value2 9 IF Operator 10 1: Answer ← Value1 + Value2 11 2: Answer ← Value1 - Value2 12 3: Answer ← Value1 * Value2 13 4: Answer ← Value1 / Value2 14 ENDCASE 15 OUTPUT "The answer is ", Value1 16 OUTPUT "Do you wish to enter more values (Yes or No)?" 17 INPUT MoreValues 18 IF MoreValues = "No" 19 THEN 20 Continue ← 1 21 ENDIF 22 UNTIL Continue = 0
- (a)Find the five errors in the pseudocode and suggest a correction for each error. Error 1 Correction Error 2 Correction Error 3 Correction Error 4 Correction Error 5 Correction[5]
- (b)The algorithm needs changing to allow only the numbers 1, 2, 3, or 4 to be entered for the input variable Operator. Write the pseudocode to perform this task and state where in the algorithm it would be located. Pseudocode Location in algorithm[5]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.
- One mark for error identified and suggested correction (Max three)
- Line 8 OUTPUT Value2 - should be INPUT Value2
- Line 9 IF Operator – should be CASE OF Operator
- Line 15 OUTPUT "The answer is ", Value1 - should be Answer
- The loop may be corrected using a number of alternative methods:
- One mark for error identified and suggested correction (Max two)
- Method 1
- Line 1 Continue ← 1 should be Continue ← 0
- Line 22 UNTIL Continue = 0 should be ENDWHILE // Line 2 WHILE Continue = 0 should be REPEAT and Line 22 UNTIL Continue = 0 should be Until Continue = 1
- OR
- Method 2
- Line 2 WHILE Continue = 0 should be REPEAT
- Line 20 Continue ← 1 should be Continue ← 0 // Line 1 Continue ← 1 should be Continue ← 0 and Line 22 UNTIL Continue = 0 should be Until Continue = 1
- OR
- Method 3
- Line 2 WHILE Continue = 0 should be WHILE Continue = 1
- Line 20 Continue ← 1 should be Continue ← 0 and Line 22 UNTIL Continue = 0 should be ENDWHILE
- One mark per bullet
- MP1 Appropriate loop (begin and end) / otherwise selection
- MP2 Testing both ends of condition
- MP3 Suitable message
- MP4 Input/re-input
- WHILE Operator < 1 OR Operator > 4 (DO)
- OUTPUT "Enter 1, 2, 3 or 4"
- INPUT Operator
- ENDWHILE
- Alternative answer
- REPEAT
- IF Operator < 1 OR Operator > 4
- THEN
- OUTPUT "Enter 1, 2, 3 or 4"
- INPUT Operator
- ENDIF
- UNTIL Operator >= 1 AND Operator <= 4
- One mark
- After line 4 / between lines 2 and 5
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