← IGCSE Computer Science Paper 2

Function

A named subroutine that returns a single value to the place where it is called, so the call can be used inside an expression, such as x ← Total(2, 3).

Part of: IGCSE Computer Science 0478 — 8.1c Procedures and Functions — Module 3

Understanding Function in IGCSE Computer Science Paper 2

What it does: Double(4) is replaced by the value it returns (8), so it can be added to 1 like any other value. RETURNS BOOLEAN states that the single value coming back will be TRUE or FALSE. A function always returns one value. A procedure does not return a value. A function call produces a value, so it can be stored in a variable with an assignment. CALL is used for procedures. The call is replaced by the returned value 5 in the expression, so 5 + 1 = 6 is output.

Common mistakes: The header must say RETURNS before the data type of the value that comes back. It should be FUNCTION Pi() RETURNS REAL. A function must send its value back to the call with RETURN. OUTPUT only displays it and nothing comes back. It should be RETURN Result.

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This module's key terms are also in a printable crossword (Modules 389–398) and word search (Modules 379–398).

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Learning

Duo

Which statement describes a function?

A named subroutine that returns a single value to where it is calledA named subroutine that never gives anything back to the program

Matches

FUNCTION Double(N : INTEGER) RETURN N * 2 ENDFUNCTION

↓

The header has no RETURNS data type, so the type of the value that comes back is not stated. It should be RETURNS INTEGER.

Applying

Meanings

Term

FUNCTION

Meaning

A named subroutine that returns a single value to the place where it is called, so the call can be used inside an expression, such as x ← Total(2, 3).

Practice

FUNCTION Total(A : INTEGER, B : INTEGER) RETURNS INTEGER

Testing

Gaps

Twice(N : INTEGER) RETURNS INTEGER

Check Answer

Spots

1FUNCTION Pi() REAL

2 RETURN 3.14

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Quick Questions

1. Which statement describes a function?

A. A named subroutine that returns a single value to where it is called ✓

B. A named subroutine that never gives anything back to the program

A function always returns one value. A procedure does not return a value.

2. Which line is a correct way to use a function call?

A. CALL Total(2, 3) ← Answer

B. Answer ← Total(2, 3) ✓

A function call produces a value, so it can be stored in a variable with an assignment. CALL is used for procedures.

3. How many values does a function return to the place where it is called?

A. One value ✓

B. Several values at once

A function returns a single value. That value then replaces the function call in the expression.

4. A function is declared like this: FUNCTION Area(W : INTEGER) RETURNS INTEGER. What does RETURNS INTEGER tell us?

A. The parameter W must be typed in by the user

B. The value returned has the data type INTEGER ✓

The data type after RETURNS is the type of the value that comes back from the function.

5. Which is valid use of a function call inside an expression?

A. Score ← Total(2, 3) + 10 ✓

B. Score ← CALL Total(2, 3) + 10

A function call can be part of an expression because it gives back a value. The word CALL is not used with functions.

Match the Pairs

FUNCTION Double(N : INTEGER) RETURNS INTEGER → Starts a function called Double that takes an integer and sends back one INTEGER value

RETURN N * 2 → Sends one value back to the place where the function was called

A function call can be used as part of an expression because it gives back one value → Answer ← Double(4) + 1

The returned value has a data type that is declared in the function header → FUNCTION IsAdult(Age : INTEGER) RETURNS BOOLEAN

FUNCTION Double(N : INTEGER) RETURN N * 2 ENDFUNCTION → The header has no RETURNS data type, so the type of the value that comes back is not stated. It should be RETURNS INTEGER.

FUNCTION Double(N : INTEGER) RETURNS INTEGER OUTPUT N * 2 ENDFUNCTION → The function uses OUTPUT instead of RETURN, so no value comes back to the call. It should be RETURN N * 2.

Key Term

FUNCTION

A named subroutine that returns a single value to the place where it is called, so the call can be used inside an expression, such as x ← Total(2, 3).

Fill in the Blanks

1. FUNCTION Twice(N : INTEGER) RETURNS INTEGER RETURN N * 2 ENDFUNCTION

Hint: Type the keyword that starts the definition of a subroutine that gives back a value.

2. FUNCTION Twice(N : INTEGER) RETURNS INTEGER RETURN N * 2 ENDFUNCTION

Hint: Type the keyword that comes before the data type of the value the function sends back.

3. FUNCTION Twice(N : INTEGER) RETURNS INTEGER RETURN N * 2 ENDFUNCTION

Hint: Type the keyword that sends the single value back to the place where the function was called.

4. FUNCTION IsBig(N : INTEGER) RETURNS BOOLEAN RETURN N > 100 ENDFUNCTION

Hint: Type the data type of the value returned. The comparison gives TRUE or FALSE.

5. x ← Total(2, 3) OUTPUT x

Hint: Type the assignment symbol that stores the value returned by the function call in the variable.

Spot the Error

Question 1

A FUNCTION Pi() REAL

B RETURN 3.14

C ENDFUNCTION

D OUTPUT Pi()

Line A — The header must say RETURNS before the data type of the value that comes back. It should be FUNCTION Pi() RETURNS REAL.

Question 2

A FUNCTION Half(N) RETURNS REAL

B Result ← N / 2

C OUTPUT Result

D ENDFUNCTION

Line C — A function must send its value back to the call with RETURN. OUTPUT only displays it and nothing comes back. It should be RETURN Result.

Question 3

A FUNCTION Sum(A,B) RETURNS REAL

B Result ← A + B

C RETURN Result

D ENDPROCEDURE

E OUTPUT Sum(2,3)

Line D — A function definition must be closed with ENDFUNCTION, not ENDPROCEDURE. ENDPROCEDURE belongs to procedures.

Question 4

A FUNCTION Sq(N) RETURNS REAL

B RETURN N * N

C ENDFUNCTION

D Area ← CALL Sq(5)

Line D — CALL is only used for procedures. A function is called by its name inside an expression, so it should be Area ← Sq(5).

Question 5

A FUNCTION Hi() RETURNS STRING

B RETURN 42

C ENDFUNCTION

D OUTPUT Hi()

Line B — The function is declared to return a STRING, but 42 is an INTEGER. The returned value must match the declared type, e.g. RETURN "Hello".

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