Subroutines
Procedures

Defining Procedures · Calling · Benefits of Subroutines

CSZoneAQA GCSE Computer Science 8525

What is a Subroutine?

Named Blocks of Code

A subroutine is a named block of code that performs a specific task. It can be called (executed) from anywhere in the program. AQA has two types: procedures (no return value) and functions (return a value).

PROCEDURE

Performs a task but does NOT return a value

FUNCTION

Performs a task AND returns a value back to the caller

Defining a Procedure in AQA

SUBROUTINE … ENDSUBROUTINE

DEFINE

SUBROUTINE greet()
OUTPUT 'Hello!'
OUTPUT 'Welcome to CSZone.'
ENDSUBROUTINE

CALL (invoke)

greet()
← Hello!
← Welcome to CSZone.

greet() ← call again
← Hello!
← Welcome to CSZone.

Key:The subroutine is defined once but can be called many times — write once, reuse everywhere.

Benefits of Subroutines

Why Use Subroutines?

Reusability — define once, call many times; no need to repeat code

Readability — main program is shorter and easier to understand

Maintainability — fix a bug in one place; change takes effect everywhere

Decomposition — break large problems into smaller, manageable tasks

Real-world analogy:A subroutine is like a recipe — write it once and follow it whenever you need it.

Exam Practice

Have a go at this question

AQA-style question

Write a procedure called printLine that outputs 20 asterisk (*) characters in a row. Then call the procedure twice from the main program.

4 marks

SUBROUTINE printLine()
  line ← ''
  FOR i ← 1 TO 20
    line ← line + '*'
  ENDFOR
  OUTPUT line
ENDSUBROUTINE

printLine()
printLine()

Key Takeaways

What to Remember

Subroutines = named blocks of reusable code

AQA syntax: SUBROUTINE name() … ENDSUBROUTINE

Benefits: reuse, readability, maintainability, decomposition

Procedures don't return a value — Functions do (covered in 3.2.10b)