Procedures vs Functions: Side by Side — Procedures and Functions | IGCSE Computer Science

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 4% of your exam marks.

stable

Rare

Stable4%

Writing and calling procedures/functions with parameters is tested in Paper 2.

This comparison appears in almost every exam paper.

Feature	Procedure	Function
Returns a value?	No	Yes
Keyword used to define	`PROCEDURE ... ENDPROCEDURE`	`FUNCTION ... ENDFUNCTION`
Uses `RETURNS` in header?	No	Yes (`RETURNS <data type>`)

Quick rule of thumb

If the sub-program's job is to DO something, write a procedure and call it with CALL.
If the sub-program's job is to PRODUCE a value, write a function and use it inside an expression.

Exam tip

Procedure vs function: what the mark scheme expects

What comes up: "Describe how a function is used in a program" or "Explain the difference between a procedure and a function" (3 marks, max three from four mark points).

Write: State all four steps in the function's lifecycle: (1) the function is invoked by its identifier, using a call statement; (2) arguments may be passed in from the calling program as parameters; (3) the function carries out its task; (4) it then returns a value back to the calling program, where it appears as part of an expression. For a procedure: the same first three steps apply, but a procedure does not return a value and is invoked with the CALL keyword rather than inside an expression.

Watch out: The most commonly dropped mark is omitting that the function returns a value to the calling program. A procedure performs a task and stops there; stating that a procedure "returns a result" is incorrect and will cost the mark for that distinction. Also note: a function is used directly inside an expression (e.g. assigned to a variable or placed in a condition), never with CALL.