AS · Practice questions · Passing parameters by value and by reference

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Six original Cambridge-style questions on passing parameters by value and by reference, with mark points.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Value in, reference out.

BYVAL: a copy; changes stay inside the subroutine; the default if nothing is stated.
BYREF: the address; the parameter is the caller's variable; the argument must be a variable.
One keyword applies to all the following parameters until the other keyword appears.
Functions take BYVAL parameters only and return one value; use BYREF in a procedure to pass back several values.

Recall

[2 marks]

Explain the difference between passing a parameter by value and by reference.

By value: a copy of the value is passed; changes made in the subroutine do not affect the original variable. ✓
By reference: the address of the variable is passed; changes made in the subroutine are made to the original variable. ✓

Trace

[4 marks]

Give the two lines of output from this program.

PROCEDURE Process(BYVAL A : INTEGER, BYREF B : INTEGER)
    A ← A + B
    B ← A * 2
ENDPROCEDURE

X ← 4
Y ← 1
CALL Process(X, Y)
OUTPUT X, " ", Y
CALL Process(Y, X)
OUTPUT X, " ", Y

First call: A = 4 (copy of X), B refers to Y; A ← 5, B ← 10, so Y = 10 and X is unchanged. ✓
First output: 4 10. ✓
Second call: A = 10 (copy of Y), B refers to X (4); A ← 14, B ← 28, so X = 28. ✓
Second output: 28 10. ✓

Write

[4 marks]

Write a procedure Order with two integer parameters that, after it is called, leaves the smaller value in the first variable and the larger value in the second.

Header: PROCEDURE Order(BYREF First : INTEGER, Second : INTEGER) (both by reference). ✓
IF First > Second THEN ... ENDIF. ✓
Swap using a local temporary variable: Temp ← First, First ← Second, Second ← Temp. ✓
ENDPROCEDURE, with Temp declared as INTEGER. ✓

Write

[5 marks]

Write a procedure Stats that takes three integers and passes back both their total and their mean. Write a call that passes 4, 9 and 8 and then outputs the results, and state what is output.

PROCEDURE Stats(BYVAL N1 : INTEGER, N2 : INTEGER, N3 : INTEGER, BYREF Total : INTEGER, Mean : REAL). ✓
Total ← N1 + N2 + N3. ✓
Mean ← Total / 3 and ENDPROCEDURE. ✓
CALL Stats(4, 9, 8, Sum, Average) then OUTPUT Sum, " ", Average, with Sum and Average declared as variables. ✓
Output: 21 7.0 (Mean is REAL). ✓

Explain

[2 marks]

Explain why a procedure, rather than a function, is used when a subroutine must pass back two values to the calling program.

A function returns only a single value (and should not use BYREF parameters). ✓
A procedure can pass back several values through BYREF parameters, which update the caller's variables. ✓

Analysis

[3 marks]

A student writes PROCEDURE AddBonus(Score : INTEGER) containing Score ← Score + 10. After CALL AddBonus(PlayerScore), PlayerScore has not changed. Explain why, and describe two different ways to correct it.

No keyword is given, so Score is passed by value: the procedure adds 10 to a copy, which is discarded. ✓
Fix 1: change the header to PROCEDURE AddBonus(BYREF Score : INTEGER) so the original variable is updated. ✓
Fix 2: make it a function, FUNCTION AddBonus(Score : INTEGER) RETURNS INTEGER that returns Score + 10, and call it with PlayerScore ← AddBonus(PlayerScore). ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.