11.3.2 Functions and Return Values

11.3.2 Functions and Return Values

A function is a named subroutine that produces one value for its caller. Because it produces a value, a function call can appear inside an assignment, calculation, output statement or Boolean expression.

This page focuses on function definitions, calls, return values and local processing. Parameters are introduced where they help explain realistic functions, while detailed parameter-passing mechanisms are developed in 11.3.3.

What Is a Function?

A function performs a defined task and sends one result back to the point from which it was called.

Simple function with no parameters

FUNCTION GetDefaultLevel() RETURNS INTEGER
    RETURN 4
ENDFUNCTION

The function does not produce output by itself. It produces the integer value 4 for use by the calling expression.

CurrentLevel ← GetDefaultLevel()

After the function returns, the assignment behaves as:

CurrentLevel ← 4

Procedure or Function?

Procedure example

PROCEDURE DisplayMenu
    OUTPUT "1. Add record"
    OUTPUT "2. Edit record"
    OUTPUT "3. Exit"
ENDPROCEDURE

CALL DisplayMenu

Function example

FUNCTION CalculateArea(Width : INTEGER,
                       Height : INTEGER) RETURNS INTEGER
    RETURN Width * Height
ENDFUNCTION

PanelArea ← CalculateArea(7, 4)

The Parts of a Function

FUNCTION CalculateArea(Width : REAL,
                       Height : REAL) RETURNS REAL
    DECLARE Area : REAL

    Area ← Width * Height

    RETURN Area
ENDFUNCTION

The Function Interface

The interface describes how another part of the program can use the function. It includes the function name, its parameters and the type of value returned.

FUNCTION CalculateArea(Width : REAL,
                       Height : REAL) RETURNS REAL

PanelArea ← CalculateArea(7.5, 4.2)

In this call, 7.5 and 4.2 are arguments supplied to the function. Detailed parameter and argument terminology continues in the next section.

How a Return Value Replaces a Function Call

FUNCTION GetBonus() RETURNS INTEGER
    RETURN 6
ENDFUNCTION

FinalScore ← 14 + GetBonus()

Using Function Calls in Expressions

A function call can be placed wherever a value of its return type would be valid.

Assignment

PanelArea ← CalculateArea(7, 4)

Arithmetic expression

TotalSpace ← ExistingSpace + CalculateArea(7, 4)

Output statement

OUTPUT "Area: ", CalculateArea(7, 4)

Boolean condition

IF IsSafeReading(CurrentReading) = TRUE
THEN
    OUTPUT "Reading accepted"
ENDIF

Argument supplied to another function

Category ← GetAreaCategory(CalculateArea(7, 4))

CALL CalculateArea(7, 4)

Function Return Types

The value returned must be compatible with the type declared in the function header.

Boolean function

FUNCTION IsSafeReading(Reading : INTEGER) RETURNS BOOLEAN
    RETURN Reading >= 20 AND Reading <= 80
ENDFUNCTION

String function

FUNCTION GetAvailability(InStock : BOOLEAN) RETURNS STRING
    IF InStock = TRUE
    THEN
        RETURN "Available"
    ELSE
        RETURN "Unavailable"
    ENDIF
ENDFUNCTION

Return Statements and Alternative Paths

A function can contain selection. Every possible execution path should produce an appropriate returned value.

FUNCTION GetTemperatureBand(Temperature : REAL)
    RETURNS STRING

    IF Temperature < 10
    THEN
        RETURN "Cold"
    ELSE
        IF Temperature <= 24
        THEN
            RETURN "Moderate"
        ELSE
            RETURN "Warm"
        ENDIF
    ENDIF
ENDFUNCTION

Executing RETURN immediately ends the current function call. Statements later in the same path are not executed.

Unreachable statement

FUNCTION GetCode() RETURNS CHAR
    RETURN 'A'
    OUTPUT "This statement cannot run"
ENDFUNCTION

Local Variables and the Return Boundary

A function may use local variables while calculating its result. These variables belong to the current function call and are not accessed directly by the main program.

FUNCTION CalculateArea(Width : REAL,
                       Height : REAL) RETURNS REAL
    DECLARE Area : REAL

    Area ← Width * Height

    RETURN Area
ENDFUNCTION

PanelArea ← CalculateArea(7.5, 4.2)

A global variable has wider availability. Using global data unnecessarily can make a function harder to test because its result may depend on state that is not visible in its interface.

When Is a Function Appropriate?

Useful design question

Using Functions Efficiently

Do not repeat the same function call when its result can be stored and reused.

Repeated calculation

IF CalculateArea(Width, Height) > 50
THEN
    OUTPUT "Large area: ", CalculateArea(Width, Height)
ENDIF

The function may calculate the same area twice.

Improved version

Area ← CalculateArea(Width, Height)

IF Area > 50
THEN
    OUTPUT "Large area: ", Area
ENDIF

Keep dependencies visible

FUNCTION CalculateCharge() RETURNS REAL
    RETURN GlobalRate * GlobalDuration
ENDFUNCTION

FUNCTION CalculateCharge(Rate : REAL,
                         Duration : REAL) RETURNS REAL
    RETURN Rate * Duration
ENDFUNCTION

Supplying the values through the interface makes the function easier to test with different data.

A Reliable Method for Tracing a Function

1. Locate the function call inside the caller’s expression.
2. Record the arguments supplied by the caller.
3. Match each argument with the corresponding parameter.
4. Enter the function and initialise its local state.
5. Execute the body in order, including any selection or repetition.
6. Identify the executed RETURN statement.
7. Replace the original call with the returned value.
8. Finish evaluating the caller’s expression.
9. Continue with the next caller statement.

Suggested trace-table columns

Worked Example: Estimating Drone Endurance

A monitoring drone stores its battery capacity in watt-hours and its average power use in watts. One function calculates the estimated flight time, and a second function creates an endurance label.

Function definitions

FUNCTION CalculateFlightMinutes(CapacityWh : REAL,
                                AveragePowerW : REAL)
    RETURNS REAL

    DECLARE Hours : REAL
    DECLARE Minutes : REAL

    Hours ← CapacityWh / AveragePowerW
    Minutes ← Hours * 60

    RETURN Minutes
ENDFUNCTION

FUNCTION GetEnduranceBand(Minutes : REAL)
    RETURNS STRING

    IF Minutes < 15
    THEN
        RETURN "Short"
    ELSE
        IF Minutes <= 30
        THEN
            RETURN "Standard"
        ELSE
            RETURN "Extended"
        ENDIF
    ENDIF
ENDFUNCTION

Main program

DECLARE CapacityWh : REAL
DECLARE AveragePowerW : REAL
DECLARE FlightMinutes : REAL
DECLARE EnduranceBand : STRING

CapacityWh ← 72
AveragePowerW ← 180

FlightMinutes ← CalculateFlightMinutes(
                    CapacityWh,
                    AveragePowerW
                )

EnduranceBand ← GetEnduranceBand(FlightMinutes)

OUTPUT "Estimated flight time: ", FlightMinutes
OUTPUT "Endurance band: ", EnduranceBand

Trace the first function

Trace the second function

Final output

Estimated flight time: 24
Endurance band: Standard

Interactive: Function Return Visualiser

Choose a scenario and trace the call, local processing, return statement and completed caller expression.

BaseScore ← 14
FinalScore ← BaseScore + GetBonus()
OUTPUT FinalScore

FUNCTION GetBonus() RETURNS INTEGER
    DECLARE Bonus : INTEGER
    Bonus ← 6
    RETURN Bonus
ENDFUNCTION

Common Mistakes and Misconceptions

Practice

FUNCTION GetLimit() RETURNS INTEGER
    RETURN 50
ENDFUNCTION

FUNCTION GetIncrease() RETURNS INTEGER
    RETURN 7
ENDFUNCTION

Result ← 18 + GetIncrease()

FUNCTION GetQuantity() RETURNS INTEGER
    RETURN "Twelve"
ENDFUNCTION

FUNCTION GetMessage() RETURNS STRING
    RETURN "Ready"
    OUTPUT "Function complete"
ENDFUNCTION

FUNCTION GetResult(Passed : BOOLEAN) RETURNS STRING
    IF Passed = TRUE
    THEN
        RETURN "Pass"
    ENDIF
ENDFUNCTION

IF CalculateVolume(Length, Width, Height) > 100
THEN
    OUTPUT CalculateVolume(Length, Width, Height)
ENDIF

AreaCategory ← GetAreaCategory(
                   CalculateArea(8, 6)
               )

FUNCTION GetStartCode() RETURNS CHAR
    RETURN 'S'
ENDFUNCTION

CurrentCode ← GetStartCode()

Result ← 18 + 7
Result ← 25

FUNCTION CalculatePerimeter(Length : INTEGER,
                            Width : INTEGER)
    RETURNS INTEGER

    RETURN 2 * (Length + Width)
ENDFUNCTION

FUNCTION IsValidPercentage(Value : INTEGER)
    RETURNS BOOLEAN

    RETURN Value >= 0 AND Value <= 100
ENDFUNCTION

FUNCTION GetQuantity() RETURNS INTEGER
    RETURN 12
ENDFUNCTION

FUNCTION GetResult(Passed : BOOLEAN) RETURNS STRING
    IF Passed = TRUE
    THEN
        RETURN "Pass"
    ELSE
        RETURN "Not passed"
    ENDIF
ENDFUNCTION

Volume ← CalculateVolume(Length, Width, Height)

IF Volume > 100
THEN
    OUTPUT Volume
ENDIF

Review