Imperative Programming (Cambridge (CIE) A Level Computer Science): Revision Note

Imperative programming (Procedural)

How do I write imperative (procedural) code?

• High-level languages such as Python, Java, and Visual Basic support the imperative style

• Imperative code can be made more organised and readable by using structured programming principles, such as dividing code into functions and procedures

  • Before continuing, ensure you’re confident with the basics of programming logic and have a good understanding of structured programming

Imperative programming – pseudocode (no procedures or functions)

• Task: Convert a temperature to:

  • Fahrenheit from Celsius

  • Celsius from Fahrenheit

  • Kelvin from Celsius

• Rules:

  • No procedures or functions

  • Use only global variables

  • Use a top-down, sequential flow (basic imperative structure)

Imperative pseudocode

// Global variables
DECLARE inputTemp : REAL
DECLARE convertedTemp : REAL
DECLARE choice : INTEGER

OUTPUT "Select conversion type:"
OUTPUT "1. Celsius to Fahrenheit"
OUTPUT "2. Fahrenheit to Celsius"
OUTPUT "3. Celsius to Kelvin"

INPUT choice
OUTPUT "Enter the temperature:"
INPUT inputTemp

IF choice = 1 THEN
    SET convertedTemp = (inputTemp * 9 / 5) + 32
    OUTPUT "Temperature in Fahrenheit: ", convertedTemp
ELSE IF choice = 2 THEN
    SET convertedTemp = (inputTemp - 32) * 5 / 9
    OUTPUT "Temperature in Celsius: ", convertedTemp
ELSE IF choice = 3 THEN
    SET convertedTemp = inputTemp + 273.15
    OUTPUT "Temperature in Kelvin: ", convertedTemp
ELSE
    OUTPUT "Invalid option selected."
ENDIF

Walkthrough – What’s happening?

• All variables are declared globally

• The entire program is written as one long block

• There is no reusability (e.g. if you want to convert again, you'd have to rerun the whole thing)

• Changes to the program will likely lead to code duplication and harder maintenance

Structured (procedural) programming – pseudocode

• Goal: Rewrite the above code using procedures or functions, local variables, and modular logic

// Global variable
DECLARE choice : INTEGER

PROCEDURE main()
    OUTPUT "Select conversion type:"
    OUTPUT "1. Celsius to Fahrenheit"
    OUTPUT "2. Fahrenheit to Celsius"
    OUTPUT "3. Celsius to Kelvin"

    INPUT choice

    IF choice = 1 THEN
        CALL convertCtoF()
    ELSE IF choice = 2 THEN
        CALL convertFtoC()
    ELSE IF choice = 3 THEN
        CALL convertCtoK()
    ELSE
        OUTPUT "Invalid option selected."
    ENDIF
ENDPROCEDURE

PROCEDURE convertCtoF()
    DECLARE inputTemp : REAL
    DECLARE result : REAL
    OUTPUT "Enter temperature in Celsius:"
    INPUT inputTemp
    SET result = (inputTemp * 9 / 5) + 32
    OUTPUT "Temperature in Fahrenheit: ", result
ENDPROCEDURE

PROCEDURE convertFtoC()
    DECLARE inputTemp : REAL
    DECLARE result : REAL
    OUTPUT "Enter temperature in Fahrenheit:"
    INPUT inputTemp
    SET result = (inputTemp - 32) * 5 / 9
    OUTPUT "Temperature in Celsius: ", result
ENDPROCEDURE

PROCEDURE convertCtoK()
    DECLARE inputTemp : REAL
    DECLARE result : REAL
    OUTPUT "Enter temperature in Celsius:"
    INPUT inputTemp
    SET result = inputTemp + 273.15
    OUTPUT "Temperature in Kelvin: ", result
ENDPROCEDURE

// Start program
CALL main()

Walkthrough – what’s improved?

• Logic is divided into reusable blocks (procedures)

• Each procedure has local variables

• The program is easier to read, maintain, and extend

• Only one global variable (choice) is used for communication

• Additional features (like repeating conversions or logging) can be added more easily

Summary