Subroutines

A subroutine is a named block of code that performs a specific task. It can be called (invoked) from any part of the program, allowing code reuse and modular design.

AQA 7517 recognises two types of subroutine:

Defining and Calling Subroutines in AQA Pseudocode

Procedure (no return value)

PROCEDURE greet(name : STRING)
    OUTPUT "Hello, " & name
ENDPROCEDURE

// Calling the procedure:
CALL greet("Ada")

Function (with return value)

FUNCTION square(n : INTEGER) RETURNS INTEGER
    RETURN n * n
ENDFUNCTION

// Calling the function:
result ← square(5)   // result = 25

Parameters and Arguments

Parameters are the variables listed in the subroutine definition — they receive values when the subroutine is called.

Arguments are the actual values passed to a subroutine when it is called.

Example: in PROCEDURE greet(name : STRING), name is the parameter. When we call CALL greet("Ada"), "Ada" is the argument.

Passing by Value vs. Passing by Reference

In AQA A-Level, passing by value is the default. Passing by reference requires explicit marking (BYREF keyword or indicated in the question).

Multiple Parameters

FUNCTION add(a : INTEGER, b : INTEGER) RETURNS INTEGER
    RETURN a + b
ENDFUNCTION

total ← add(3, 7)   // total = 10

Benefits of Using Subroutines

• Code reuse — write once, call many times; no code duplication
• Modularity — break complex problems into smaller, manageable parts (decomposition)
• Readability — well-named subroutines make code self-documenting
• Easier debugging — test each subroutine in isolation
• Easier maintenance — change a subroutine in one place to update all callers
• Abstraction — callers don't need to know the implementation details

Scope in Subroutines

Parameters and local variables inside a subroutine have local scope — they exist only within the subroutine and are destroyed when it returns. Global variables declared outside subroutines are accessible everywhere.