A Level Computer Science: Fundamentals of Programming — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Fundamentals of Programming revision notes

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Section A

1. Name five primitive data types and give an example value of each. [5]

2. Explain the difference between a variable and a constant, and state one benefit of using constants. [3]

Section B

3. Explain the difference between a procedure and a function, and state when each should be used. [4]

4. Explain the difference between passing a parameter by value and by reference, and give a situation suited to each. [6]

5. Write pseudocode for a function that takes an array of integers and returns the largest value. [6]

6. Explain the difference between local and global scope, and give two reasons why global variables should be avoided. [5]

7. Explain what recursion is, state the two components every recursive routine must have, and give one drawback compared with iteration. [5]

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Section C

8. A program uses both WHILE and REPEAT UNTIL loops.

(a) Distinguish between WHILE and REPEAT UNTIL in terms of when the condition is tested. [2]

(b) Explain the consequence of this difference for how many times the loop body can execute. [2]

9. Trace the following pseudocode and state the output.

total = 0
count = 1
WHILE count <= 4
    total = total + (count * count)
    count = count + 1
ENDWHILE
OUTPUT total

Draw a trace table showing the value of count, count*count and total on each pass, then give the output. [5]

10. A program reads data from a file.

(a) State the three-step pattern every file-handling operation follows. [3]

(b) Explain one consequence of failing to close a file after use. [2]

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Answers

1. Integer — 42 [1]. Real/float — 3.14 [1]. Boolean — TRUE [1]. Character — ‘A’ [1]. String — “hello” [1].

2. A variable’s value can change during execution; a constant’s value is fixed at declaration and cannot be changed [1] [1]. Benefit: the value is defined in one place, so changing it — for example a VAT rate — requires only one edit and cannot be accidentally overwritten, which reduces errors and improves readability [1].

3. A procedure carries out a task but does not return a value; a function returns a single value to the calling statement [1] [1]. Use a procedure where the purpose is an action, such as printing a report [1]; use a function where the purpose is to compute a value used in an expression, such as calculating an average [1].

4. By value — a copy of the data is passed to the subroutine [1]; any changes made affect only the copy, so the original is unchanged [1]. By reference — the memory address of the data is passed [1], so changes made inside the subroutine alter the original variable [1]. By value suits a subroutine that only needs to read the data, since it protects the original from accidental modification [1]. By reference suits cases where the subroutine must modify the caller’s data or where the data is large, since copying a large array wastes memory and time [1].

5.

FUNCTION FindMax(numbers : ARRAY OF INTEGER) RETURNS INTEGER
    max ← numbers[0]
    FOR i ← 1 TO LENGTH(numbers) - 1
        IF numbers[i] > max THEN
            max ← numbers[i]
        ENDIF
    NEXT i
    RETURN max
ENDFUNCTION

Correct function header with parameter and return type [1]; initialising max to the first element rather than to zero [1]; loop covering every remaining element [1]; correct comparison [1]; assignment when a larger value is found [1]; RETURN statement outside the loop [1].

6. A local variable exists only within the subroutine in which it is declared and is destroyed when that subroutine ends [1]; a global variable is declared outside all subroutines and is accessible everywhere in the program [1]. Globals should be avoided because any part of the program can change them, so a bug can be introduced anywhere and is very hard to trace [1]; they also prevent subroutines from being self-contained and reusable, since the subroutine depends on something outside itself [1]; and they occupy memory for the whole run rather than only while needed [1].

7. Recursion is a technique in which a subroutine calls itself to solve a smaller instance of the same problem [1]. Every recursive routine must have a base case that stops the recursion [1] and a general case that calls itself with a value moving towards the base case [1]. Drawback: each call adds a stack frame to the call stack, so recursion uses considerably more memory than iteration and risks a stack overflow if the depth is large [1] [1].

8. (a) WHILE tests its condition before the first pass through the loop body [1]; REPEAT UNTIL tests its condition after the body has executed [1].

(b) A WHILE loop’s body may execute zero times if the condition is false from the start [1]; a REPEAT UNTIL loop’s body always executes at least once, since the first test happens only after the body has already run [1].

9.

count	count*count	total
1	1	1
2	4	5
3	9	14
4	16	30

[4, one per correctly traced row]. The loop then tests count = 5, which fails, so the output is 30 [1] — the sum of the first four square numbers.

10. (a) Open the file [1]; read or write to it [1]; close it [1].

(b) Any one: the file may remain locked, preventing other programs from accessing it [1]; buffered data may not be flushed to disk, risking data loss [1]; or system resources allocated to the open file remain unnecessarily reserved [1].

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Where marks are usually lost

• Initialising the maximum to 0, which fails for arrays of negative numbers.
• Saying a function “does something” without returning a value.
• Confusing pass by reference with returning a value.
• Omitting the base case when describing recursion.
• Confusing which loop type tests its condition before versus after the body.
• Forgetting to close a file after reading or writing to it.