IGCSEComputer ScienceCambridge (CIE)Revision Notes8. ProgrammingProgramming ConceptsProcedures & Functions

Procedures & Functions (Cambridge (CIE) IGCSE Computer Science)

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Robert Hampton

Written by: Robert Hampton

Reviewed by: James Woodhouse

What are functions and procedures?

  • Functions and procedures are a type of sub program, a sequence of instructions that perform a specific task or set of tasks

  • Procedures and functions are defined at the start of the code

  • Sub programs are often used to simplify a program by breaking it into smaller, more manageable parts

  • Sub programs 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 sub program

    • Parameters can be variables or values and they are located in brackets after the name of the sub program

    • Example: FUNCTION TaxCalculator(pay,taxcode) OR PROCEDURE TaxCalculator(pay,taxcode)

  • Sub programs can have multiple parameters

  • To use a sub program you 'call' it from the main program

What's the difference between a function and procedure?

  • A Function returns a value whereas a procedure does not

Procedures

  • Procedures are defined using the PROCEDURE keyword in pseudocode or def keyword in Python

  • A procedure can be written as:


    PROCEDURE <identifier>
    <statements>
    ENDPROCEDURE

  • Or if parameters are being used:

    PROCEDURE <identifier>(<param1>:<data type>, <param2>:<data type>...)
    <statements>
    ENDPROCEDURE

Creating a procedure

Pseudocode

PROCEDURE calculate_area(length: INTEGER, width: INTEGER)

  area ← length * width

   OUTPUT "The area is " + area

END PROCEDURE

Python

def ageCheck(age):

if age > 18:

print("You are old enough")

else:

print("You are too young")

  • To call a procedure, it can be written as:


    CALL <identifier>
    CALL <identifier>(Value1,Value2...)

Calling a procedure

Pseudocode

CALL Calculate_area

CALL Calculate_area(5,3)

Python

ageCheck(21)

print(ageCheck(21))

Examples

  • A Python program using procedures to display a menu and navigate between them

  • Procedures are defined at the start of the program and the main program calls the first procedure to start

  • In this example, no parameters are needed

Procedures

# Procedure definition

def main_menu():

# Outputs the option
  print("1. Addition")
  print("2. Subtraction")
  print("3. Multiplication")
  print("4. Division")
  print("5. Exit")

 # Asks the user to enter their choice

choice = int(input("Enter your choice: "))
  if choice == 1:
    addition()
  elif choice == 2:
    subtraction()
  elif choice == 3:
    multiplication()
  elif choice == 4:
    division()
  elif choice == 5:
    exit()

# Procedure definition

def addition():
  num1 = int(input("Enter the first number: "))
  num2 = int(input("Enter the second number: "))
  print(num1 + num2)

def subtraction():
  num1 = int(input("Enter the first number: "))
  num2 = int(input("Enter the second number: "))
  print(num1 - num2)

def multiplication():
  num1 = int(input("Enter the first number: "))
  num2 = int(input("Enter the second number: "))
  print(num1 * num2)

def division():
  num1 = int(input("Enter the first number: "))
  num2 = int(input("Enter the second number: "))
  print(num1 / num2)

# Main program
main_menu() # Calls the main_menu procedure

Functions

  • Functions are defined using the FUNCTION keyword in pseudocode or def keyword in Python

  • Functions always return a value so they do not use the CALL function, instead they are called within an expression

  • A function can be written as:


    FUNCTION <identifier> RETURNS <data type>
    <statements>
    ENDFUNCTION

  • Or if parameters are being used:

    FUNCTION <identifier>(<param1>:<data type>, <param2>:<data type>...) RETURNS <data type>
    <statements>
    ENDFUNCTION

Creating and using a function

Pseudocode

FUNCTION calculate_area(length: INTEGER, width: INTEGER)

area ← length * width

   RETURN area

ENDFUNCTION

// Output the value returned from the function

OUTPUT(calculate_area(5,3))

Python

def squared(number):

squared = number^2

return squared

// Output the value returned from the function

print(squared(4))

Examples

  • A Python program using a function to calculate area and return the result

  • Two options for main program are shown, one which outputs the result (# 1) and one which stores the result so that it can be used at a later time (# 2)

Functions

# Function definition, length and width are parameters

def area(length, width):
  area = length * width # Calculate area
  return area # Return area

# Main program #1
length = int(input("Enter the length: "))
width = int(input("Enter the width: "))
print(area(length, width))

# Main program #2
length = int(input("Enter the length: ")) length
width = int(input("Enter the width: "))
area = area(length, width) # Stores the result of the function in a variable
print("The area is " + str(area) + " cm^2")

Worked Example

An economy-class airline ticket costs £199. A first-class airline ticket costs £595.

(A) Create a function, flightCost(), that takes the number of passengers and the type of ticket as parameters, calculates and returns the price to pay.

You do not have to validate these parameters

You must use :

  • a high-level programming language that you have studied [4]

(B) Write program code, that uses flightCost(), to output the price of 3 passengers flying economy.

You must use :

  • a high-level programming language that you have studied [3]

How do I answer this question?

(A)

  • Define the function, what parameters are needed? where do they go?

  • How do you calculate the price?

  • Return the result

(B)

  • How do you call a function?

  • What parameters does the function need to return the result?

Answers

Part

Python

A

def flightCost(passengers, type):

if type == "economy":

cost = 199 * passengers

elif type == "first":

cost = 595 * passengers

return cost

B

print(flightCost("economy", 3)

OR

x = flightCost("economy", 3)

print(x)

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    Author
    Reviewer
    Robert Hampton

    Author: Robert Hampton

    Expertise: Computer Science Content Creator

    Rob has over 16 years' experience teaching Computer Science and ICT at KS3 & GCSE levels. Rob has demonstrated strong leadership as Head of Department since 2012 and previously supported teacher development as a Specialist Leader of Education, empowering departments to excel in Computer Science. Beyond his tech expertise, Robert embraces the virtual world as an avid gamer, conquering digital battlefields when he's not coding.

    James Woodhouse

    Author: James Woodhouse

    Expertise: Computer Science

    James graduated from the University of Sunderland with a degree in ICT and Computing education. He has over 14 years of experience both teaching and leading in Computer Science, specialising in teaching GCSE and A-level. James has held various leadership roles, including Head of Computer Science and coordinator positions for Key Stage 3 and Key Stage 4. James has a keen interest in networking security and technologies aimed at preventing security breaches.