Procedures & Functions (CIE IGCSE Computer Science)

Procedures and functions are types of subroutines. They are a sequence of instructions that perform a specific task or set of tasks

• Subroutines are often used to simplify a program by breaking it into smaller, more manageable parts. They can be used to:
  • Avoid duplicating code and can be reused throughout a program,
  • Improve the readability and maintainability of code,
  • Perform calculations, to retrieve data, or to make decisions based on input.
• Parameters are values that are passed into a subroutine. They can:
  • Be used to customise the behaviour of a subroutine,
  • Be of different types,
  • Have default values, which are used if no value is passed in,
  • Be passed by value or by reference.
    • When a parameter is passed by value, a copy of the parameter is made and used within the subroutine
    • When a parameter is passed by reference, the original parameter is used within the subroutine, and any changes made to the parameter will be reflected outside of the subroutine
• Subroutines can have multiple parameters

What's the difference between a procedure and function?

Functions return a value whereas a procedure does not.

Defining and Calling a Subroutine

A subroutine only needs to be defined once to set it up. After this you can call it as many times as needed to use it.

• Procedures are defined using the PROCEDURE keyword in pseudocode, def keyword in Python, public in Java and Sub keyword in Visual Basic
• Procedures can be called from other parts of the program using their name

Pseudocode example:

PROCEDURE calculate_area(length: INTEGER, width: INTEGER)

  area ← length * width

  OUTPUT "The area is " + area

END PROCEDURE

To call the procedure you would use the following pseudocode:

calculate_area(5,3)

Python example:

def calculate_area(length, width):

    area = length * width

    print("The area is ", area)

calculate_area(5,3)

Java example:

public class Main {
  static void calculateArea(int length, int width) {
    int area = length * width;
    System.out.println("The area is " + area);
  }

  public static void main(String[] args) {
    calculateArea(3,6);
  }
}

Visual Basic example:

Sub CalculateArea(length As Integer, width As Integer)

    Dim area As Integer = length * width

    Console.WriteLine("The area is " & area)

End Sub

CalculateArea(5, 3)

• Functions are defined using the FUNCTION keyword in pseudocode, def keyword in Python, and Function keyword in Visual Basic and Java
• Functions return a value using the RETURN keyword in pseudocode, return statement in Python, and function name in Visual Basic and Java
• Functions can be called from other parts of the program using their name, and their return value can be stored in a variable

Pseudocode example:

FUNCTION calculate_area(length: INTEGER, width: INTEGER)

  area ← length * width

  RETURN area

END PROCEDURE

To output the value returned from the function you would use the following pseudocode:

OUTPUT(calculate_area(5,3))

Python example:

def calculate_area(length, width):

    area = length * width

    return area

print(calculate_area(5,3))

Java example:

public class Main {
  static int calculateArea(int length, int width) {
    int area = length * width;
    return area;
  }

  public static void main(String[] args) {
    System.out.println(calculateArea(3,6));
  }
}

Visual Basic example:

CalculateArea(5, 3)

Sub CalculateArea(length As Integer, width As Integer)

    Dim area As Integer = length * width

    Return area

End Sub