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Practise 37 exam-style questions on Trace tables for Cambridge IGCSE Computer Science (Paper 2). Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.
Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.
Look at this algorithm:
X ← 29
Y ← 4
X ← MOD(X, Y) + DIV(X, Y)
OUTPUT XState the output from this algorithm.
8
29 divided by 4 is 7 remainder 1, so DIV(29, 4) is 7 and MOD(29, 4) is 1. The new value of X is 1 + 7 = 8.
8
The array Prices holds 2.50, 3.75, 1.20 and 4.10 in Prices[1] to Prices[4].
Total ← 0
FOR Index ← 1 TO 4
Total ← Total + Prices[Index]
NEXT Index
Average ← ROUND(Total / 4, 2)
OUTPUT "Average price: ", AverageGive the values that Total takes during a dry run, and the output.
Total: 0, 2.5, 6.25, 7.45, 11.55
Output: Average price: 2.89
Total adds one price on each pass of the loop. After the loop, 11.55 ÷ 4 = 2.8875, and ROUND(..., 2) gives 2 decimal places, so 2.89 is output. A common slip is to truncate to 2.88 instead of rounding.
Total: 0, 2.5, 6.25, 7.45, 11.55
Accept 2.50 and 0.0 etc.
Output: Average price: 2.89
Do not accept 2.8875 or 2.88.
The trace table shown has been started for this algorithm, with the input Cat42x.
OUTPUT "Enter a password"
INPUT Password
Digits ← 0
FOR Index ← 1 TO LENGTH(Password)
Character ← SUBSTRING(Password, Index, 1)
IF Character >= "0" AND Character <= "9"
THEN
Digits ← Digits + 1
ENDIF
NEXT Index
IF Digits >= 2
THEN
OUTPUT "Strong"
ELSE
OUTPUT "Weak"
ENDIFGive the values that would be written in the rest of the Index, Character, Digits and OUTPUT columns.
Index: 1, 2, 3, 4, 5, 6
Character: C, a, t, 4, 2, x
Digits: 1, 2
OUTPUT: Strong
The loop runs once for each of the 6 characters, and SUBSTRING takes one character at a time. Only the characters 4 and 2 are between "0" and "9", so Digits goes up twice. After the loop, Digits >= 2 is TRUE, so Strong is output.
Index: 1, 2, 3, 4, 5, 6 with Character: C, a, t, 4, 2, x
Digits: 1, 2 (when Character is 4 and then 2)
OUTPUT: Strong
The trace table shown has been started for this algorithm. The inputs are 23, 17, 31, 19 and -1, in that order.
Count ← 0
INPUT Reading
Min ← Reading
Max ← Reading
WHILE Reading <> -1 DO
Count ← Count + 1
IF Reading < Min
THEN
Min ← Reading
ENDIF
IF Reading > Max
THEN
Max ← Reading
ENDIF
INPUT Reading
ENDWHILE
OUTPUT Count, " ", Min, " ", MaxGive the values that would be written in the rest of each column, in order.
Reading: 17, 31, 19, -1
Count: 1, 2, 3, 4
Min: 17
Max: 31
OUTPUT: 4 17 31
The first reading sets both Min and Max to 23. Inside the loop each reading is counted and compared, then the next reading is input at the end of the pass. When -1 is input, the WHILE condition is false, so -1 is never counted or treated as the minimum - this is why the rogue value is tested before it is processed.
Reading: 17, 31, 19, -1
Count: 1, 2, 3, 4
Do not accept 5.
Min: 17
Do not accept -1.
Max: 31
OUTPUT: 4 17 31
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