Subroutines: Functions and Procedures

This lesson covers subroutines — both functions and procedures — as required by OCR J277 Section 2.3. Subroutines are reusable blocks of code that perform a specific task. Understanding how to define, call, and use subroutines with parameters and return values is essential for the OCR GCSE Computer Science exam.

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What Is a Subroutine?

A subroutine is a named block of code that performs a specific task. Instead of writing the same code multiple times, you can write it once as a subroutine and call it whenever needed.

OCR distinguishes between two types of subroutine:

Type	Returns a value?	OCR Keyword	Use
Function	Yes	function...endfunction	Calculates and returns a result
Procedure	No	procedure...endprocedure	Performs an action (e.g. displays output)

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Procedures

A procedure performs a task but does not return a value. It might display output, modify data, or perform an action.

OCR Pseudocode:

procedure greet(name)
    print("Hello, " + name + "!")
    print("Welcome to the system.")
endprocedure

// Calling the procedure
greet("Alice")
greet("Bob")

Python:

def greet(name):
    print("Hello, " + name + "!")
    print("Welcome to the system.")

# Calling the function (Python uses 'def' for both)
greet("Alice")
greet("Bob")

Output:

Hello, Alice!
Welcome to the system.
Hello, Bob!
Welcome to the system.

OCR Exam Tip: In OCR pseudocode, procedures use procedure...endprocedure and functions use function...endfunction. In Python, both use def. In the exam, use the correct OCR keywords if writing pseudocode.

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Functions

A function performs a task and returns a value using the return keyword.

OCR Pseudocode:

function calculateArea(length, width)
    area = length * width
    return area
endfunction

// Calling the function and storing the result
result = calculateArea(5, 3)
print("Area: " + str(result))    // Output: Area: 15

Python:

def calculate_area(length, width):
    area = length * width
    return area

result = calculate_area(5, 3)
print("Area:", result)    # Output: Area: 15

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Parameters and Arguments

Parameters are the variables listed in the subroutine definition. Arguments are the actual values passed when the subroutine is called.

function add(a, b)       // a and b are PARAMETERS
    return a + b
endfunction

result = add(5, 3)       // 5 and 3 are ARGUMENTS

Multiple Parameters

function calculateBMI(weight, height)
    bmi = weight / (height * height)
    return bmi
endfunction

myBMI = calculateBMI(70, 1.75)
print("BMI: " + str(myBMI))

def calculate_bmi(weight, height):
    bmi = weight / (height * height)
    return bmi

my_bmi = calculate_bmi(70, 1.75)
print(f"BMI: {my_bmi:.1f}")

OCR Exam Tip: The distinction between parameters and arguments is a common exam question. Parameters are in the definition; arguments are in the call. Think of parameters as placeholders and arguments as the actual values.

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Local and Global Variables

Local Variables