Procedures & Functions (Cambridge International IGCSE Computer Science)

What are functions and procedures?

Exam Hint IGCSE 0478 Paper 2 assesses your ability to define, use, and explain functions, procedures, and parameters using both pseudocode and Python. This page shows only examinable structures, phrasing, and examples—no fluff.

• 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) ORPROCEDURE 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

Spec point — "6 (a) Understand what is meant by procedures, functions and parameters (b) Define and use procedures and functions, with or without parameters"

Procedures

• Procedures are defined using the PROCEDURE keyword in pseudocode or **def **keyword in Python
• A procedure can be written as: PROCEDURE ** ** ENDPROCEDURE
• Or if parameters are being used:

PROCEDURE (:, :...) ENDPROCEDURE

• To call a procedure, it can be written as: CALL CALL (Value1,Value2...)

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

Functions

Spec point — "6 (a) Understand what is meant by procedures, functions and parameters (b) Define and use procedures and functions, with or without parameters"

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 RETURNS ** ** ENDFUNCTION
• Or if parameters are being used:

FUNCTION (:, :...) RETURNS ENDFUNCTION

Exam Hint In pseudocode, never write CALL calculate_area() for a function. Functions are called in expressions like: OUTPUT calculate_area(5,3) Use CALL* only with procedures that do not return a value.*

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)

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
B	print(flightCost("economy", 3) OR x = flightCost("economy", 3) print(x)

Exam Hint in pseudocode, you can get marks just for using the correct structure:

• Starts with FUNCTION or PROCEDURE* [1 mark]
• Correct use of parameters and RETURN (functions) or OUTPUT* (procedures) [1 mark]
• *Called correctly from main program [1 mark]