Subroutines — Procedures & Functions

Spec & Goals 3 min

AQA Spec 3.2.10 · Subroutines (procedures and functions)

By the end of this lesson you can:

1. Define a subroutine and explain how it is called by name.
2. State the difference between a procedure and a function that returns a value.
3. Write a subroutine with parameters in AQA pseudo-code and in Python.

Warm-Up 5 min

Last lesson you used string handling — LEN, SUBSTRING and joining text together.

Quick starter

A program calculates the area of three different rooms. The same three lines of code are copied out three times. Name one problem this causes.

Key Concept — naming a block of code 14 min

A subroutine is a named block of code you can call by its name whenever you need it. The program runs the block, then carries on from where it was called.

Two kinds: procedure and function

AQA splits subroutines into two kinds:

So a function gives you a value back that you can use; a procedure simply does its job. A function is called inside an expression, such as total ← area(4, 5).

Parameters

A subroutine listing such as SUBROUTINE area(width, height) declares two parameters. When you call area(4, 5), the values 4 and 5 are passed in.

Why use subroutines

• Reuse — write the code once, call it many times.
• Readability — a sensible name explains what a block does.
• Decomposition — break a big problem into smaller named parts.
• Easier testing — test each small subroutine on its own.

Worked Example — an area function and a greeting procedure 12 min

Problem: write a function that returns the area of a rectangle, and a simple procedure that prints a welcome message.

AQA pseudo-code

SUBROUTINE area(width, height)
  RETURN width * height
ENDSUBROUTINE

SUBROUTINE greet()
  OUTPUT 'Selamat datang to the hall booking system'
ENDSUBROUTINE

# Call the function inside an expression
OUTPUT area(4, 5)

# Call the procedure on its own
greet()

The call area(4, 5) passes 4 and 5 in as the parameters. The function returns 20, so OUTPUT area(4, 5) displays 20. The procedure greet() returns nothing — it just prints its message.

The same in Python

def area(width, height):
    return width * height

def greet():
    print("Selamat datang to the hall booking system")

# Call the function inside an expression
print(area(4, 5))

# Call the procedure on its own
greet()

Try It Yourself 12 min

📝 Exam Practice 10 min

Answer the way the examiner expects — the command word and the marks tell you how much to write.

Recap & Key Terms 3 min

A subroutine is a named block of code you call by name. A procedure carries out a task; a function carries out a task and returns a value. Parameters pass data in. Subroutines aid reuse, readability, decomposition and testing.

Subroutine

A named block of code that can be called by name from elsewhere in a program.

Procedure

A subroutine that carries out a task but does not return a value.

Function

A subroutine that carries out a task and returns a value using RETURN.

Parameter

A value passed into a subroutine so it can work on different data each time.

Return value

The value a function sends back to the point where it was called.

Homework 1 min

Task (≤ 15 min): In AQA pseudo-code, write a function cube(n) that returns n multiplied by itself three times, then call it to output the cube of 3.

SUBROUTINE cube(n)
  RETURN n * n * n
ENDSUBROUTINE

OUTPUT cube(3)