A-Level Computer Science 9618 / Topic 11 / AS

Write it once, call it often.

Structured programs are built from small named subroutines. A procedure is called as a statement of its own; a function returns a single value that replaces the call in an expression. Learn to define and call both, use the exact terminology examiners expect, and write efficient pseudocode.

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The key idea

A procedure is a named block of code that performs a task; it may have none, one or more parameters and is run with a CALL statement. A function also performs a task but returns a single value: its header ends RETURNS data type, it uses RETURN, and it is used inside an expression, where the return value replaces the call. The values passed in a call are arguments; the identifiers in the header that receive them are parameters. Subroutines give modular, reusable code that is easier to test and maintain.

Fig. 1: control passes to the subroutine and back; only a function brings a value back with it

Section 01

Defining a procedure.

Use a procedure when a task is needed in more than one place, or to break a large problem into separate modules, and the task does not need to hand back a single value (it outputs something, or changes data). Step through the lab's procedure and function programs to watch control jump to the subroutine and back.

PROCEDURE ShowLine(Length : INTEGER, Symbol : CHAR)
    DECLARE Line : STRING
    DECLARE Count : INTEGER
    Line ← ""
    FOR Count ← 1 TO Length
        Line ← Line & Symbol
    NEXT Count
    OUTPUT Line
ENDPROCEDURE

PROCEDURE ShowBanner()
    CALL ShowLine(20, '=')
    OUTPUT "Weekly quiz results"
    CALL ShowLine(20, '=')
ENDPROCEDURE

CALL ShowBanner()
CALL ShowLine(5, '*')

No parameters: PROCEDURE ShowBanner() is called with CALL ShowBanner().
With parameters: the arguments in the CALL must match the parameters in number, order and data type.
A complete statement: a procedure call stands on a line of its own, starting with CALL.
Local variables: Line and Count are declared inside the procedure, so they exist only while it runs and cannot clash with variables elsewhere.

Worked example Follow the calls

State exactly what the last two lines of the program above output.

CALL ShowBanner() runs ShowBanner, which first calls ShowLine with Length 20 and Symbol '='.
ShowLine joins '=' to Line 20 times and outputs a row of 20 = signs.
ShowBanner then outputs the title and calls ShowLine(20, '=') again.
Control returns to the main program: CALL ShowLine(5, '*') outputs *****.

Answer: A row of 20 = signs, the line Weekly quiz results, another row of 20 = signs, then *****

Your turn

Write the statement that outputs a row of 12 hyphens using ShowLine.

CALL ShowLine(12, '-'): the arguments 12 and '-' are matched in order to the parameters Length and Symbol.

Examiner trap

A procedure does not return a value, so X ← ShowLine(5, '*') is wrong. If the main program needs a result back, write a function instead.

Section 02

Defining a function.

Use a function when the subroutine must work out one value for the code that calls it. A function call is not a complete statement: it is part of an expression, and when RETURN runs, the returned value replaces the call and the expression is then evaluated. CALL is never used with a function.

FUNCTION CountChar(Text : STRING, Target : CHAR) RETURNS INTEGER
    DECLARE Count : INTEGER
    DECLARE Index : INTEGER
    Count ← 0
    FOR Index ← 1 TO LENGTH(Text)
        IF MID(Text, Index, 1) = Target THEN
            Count ← Count + 1
        ENDIF
    NEXT Index
    RETURN Count
ENDFUNCTION

Total ← CountChar("banana", 'a') * 2
IF CountChar(Word, 'e') > 2 THEN
    OUTPUT "Lots of e"
ENDIF

Header: FUNCTION, the name, the parameters with their types, then RETURNS and the type of the value returned.
RETURN: sends the value back. If RETURN runs part-way through the function, the function ends immediately and the remaining lines are skipped.
Where it is used: on the right of an assignment, in an OUTPUT, or in a condition, as above.

Worked example Trace a function call

Trace Total ← CountChar("banana", 'a') * 2 and give the final value of Total.

The function runs with Text = "banana", Target = 'a'; LENGTH(Text) is 6, so Index goes 1 to 6.
Characters: b, a, n, a, n, a. Count goes up at Index 2, 4 and 6, so Count = 3.
RETURN 3: the value 3 replaces the call, so the expression becomes 3 * 2.
Total ← 6.

Answer:

Index	MID(Text, Index, 1)	Count
		0
1	b	0
2	a	1
3	n	1
4	a	2
5	n	2
6	a	3

RETURN 3, so Total = 6.

Your turn

What does CountChar("Mississippi", 's') return?

Examiner trap

CALL CountChar("banana", 'a') loses the mark: a function is never CALLed, and its return value would be thrown away.

Stage 1 · Learn

Check what the sim just showed you

Four quick checks. Each correct answer earns XP and records this skill on your profile.

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Which statement correctly runs the procedure ShowLine?

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What is the main difference between a function and a procedure?

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In Floor ← Area(4.0, 2.5), what are 4.0 and 2.5?

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In the lab, PROCEDURE Stars(Number : INTEGER) is run by CALL Stars(3). What is Number?

Section 03

The terminology.

Questions often show a subroutine and ask you to identify its parts, so learn these words precisely. Then use subroutines and sensible loop choices to write efficient pseudocode.

Term	Meaning	In `FUNCTION Area(Length : REAL, Width : REAL) RETURNS REAL`
Header	the first line: the subroutine's name, its parameters and (for a function) the return type	the whole line
Interface	how data passes in and out: the parameters with their data types and order, and the return type	two REALs in, one REAL out
Parameter	an identifier in the header that receives a value when the subroutine is called	Length, Width
Argument	the actual value or variable passed in the call	4.0 and 2.5 in `Floor ← Area(4.0, 2.5)`
Return value	the single value a function sends back to replace the call	10.0

// efficient: stops as soon as the item is found
FUNCTION FindItem(SearchItem : INTEGER) RETURNS INTEGER
    // Data is a global ARRAY[1:1000] OF INTEGER
    DECLARE Index : INTEGER
    Index ← 1
    WHILE Index <= 1000
        IF Data[Index] = SearchItem THEN
            RETURN Index
        ENDIF
        Index ← Index + 1
    ENDWHILE
    RETURN -1
ENDFUNCTION

Worked example Make it efficient

A program checks a list of 1000 values using a FOR loop from 1 to 1000 that sets a flag when the item is found, and the same 6 lines of output code are copied in three places. Suggest how to make the pseudocode more efficient.

Replace the FOR loop with a conditional loop (or RETURN inside a function, as in FindItem) so that searching stops as soon as the item is found.
If the item is at position 3, the efficient version makes 3 comparisons instead of 1000.
Put the repeated output code into one procedure and CALL it three times: less code, and a change is made in one place.
Also avoid working out the same value again inside a loop: calculate it once before the loop and store it in a variable.

Answer: Stop the loop early, and replace duplicated code with a procedure call

Your turn

In Floor ← Area(4.0, 2.5), identify the arguments and give the return value, assuming Area returns Length * Width.

Examiner trap

Do not mix up parameter and argument. The parameter is the name in the header (Length); the argument is what is passed in the call (4.0).

Stage 2 · Exam

Exam-style questions

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CountChar is a function returning an INTEGER. Which statement is NOT a valid use of it?

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When is a function more appropriate than a procedure?

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What is meant by the interface of a subroutine?

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Square(N) returns N * N. Total is 10. What is Total after Total ← Total + Square(4)?

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Six original Cambridge-style questions

Defining and calling procedures and functions in 9618 pseudocode, using the correct terminology, tracing function calls and writing efficient pseudocode.