Procedures & Functions (Cambridge (CIE) A Level Computer Science): Revision Note

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

• A procedure can be written as:

PROCEDURE <identifier>()
  <statement(s)>
ENDPROCEDURE

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

• The identifier is used to call the procedure

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

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

• When parameters are used, value1, value2... must be of the correct data type and in the same sequence

Example code

PROCEDURE GreetUser(Name : STRING)
    OUTPUT "Hello, ", Name, "!"
ENDPROCEDURE

DECLARE UserName : STRING

OUTPUT "Enter your name: "
INPUT UserName

CALL GreetUser(UserName)

• PROCEDURE GreetUser defines the procedure, which takes one parameter: Name

• The procedure prints a personalised greeting

• The main program asks for the user’s name and then calls the procedure using CALL

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>
  <statement(s)>
ENDFUNCTION

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

Example code

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))

• Defines a function called calculate_area

• It takes two parameters: length and width, both of type INTEGER

• A local variable area is declared and assigned the result of length × width

• The function returns the result to wherever it was called from

Parameter passing

What is parameter passing?

• Parameter passing is the method by which values or references are given to procedures or functions so they can use or modify data during execution

• When writing procedures, you can control how parameters are passed using:

  • BYVAL – pass a copy of the value (default behaviour)

  • BYREF – pass the actual variable, so changes inside the procedure affect the original

• If no keyword is used, it is assumed to be BYVAL

• Functions should not use BYREF parameters – only procedures should

Example – Swapping two values (Procedure with BYREF)

PROCEDURE Swap(BYREF X : INTEGER, Y : INTEGER)
    DECLARE Temp : INTEGER
    Temp ← X
    X ← Y
    Y ← Temp
ENDPROCEDURE

• X is passed by reference, so it can be changed

• Y is passed by value, so any change to Y inside the procedure does not affect the original value