In this guide:
Exam Strategy: Top-Down Design
Cambridge examiners award high marks for Top-Down Design (also known as step-wise refinement). This is the process of breaking a large, complex algorithm down into smaller, manageable subroutines. By using Functions and Procedures, you prove to the examiner that you understand modular programming, making your code easier to read, test, and debug.
Python vs. Pseudocode Subroutines
If you code in Python, you use the def keyword to create every single subroutine. However, Cambridge pseudocode strictly forces you to categorize your subroutines into two different types depending on whether they return data or not.
| Concept | 🐍 Python (Wrong for Exams) | 📘 Cambridge Pseudocode (Correct) |
|---|---|---|
| Procedure (No Return) | def show_menu(): print("Menu") |
PROCEDURE ShowMenu() OUTPUT "Menu" ENDPROCEDURE |
| Function (Returns Value) | def get_max(a, b): return a |
FUNCTION GetMax(a: INTEGER) RETURNS INTEGER RETURN a ENDFUNCTION |
| Calling a Procedure | show_menu() | CALL ShowMenu() |
1. Procedures vs. Functions
In standard pseudocode, subroutines are split into two specific categories:
2. Creating and Calling a Procedure
Procedures are defined using the PROCEDURE keyword and executed using the CALL keyword.
PROCEDURE ShowMenu()
OUTPUT "1. Start"
OUTPUT "2. Quit"
ENDPROCEDURE
// The Main Program executing the procedure:
CALL ShowMenu()3. Creating a Function
Functions require you to specify the data type they will send back using RETURNS. Inside the function, you use the RETURN keyword to pass the data back. You don't use CALL; instead, you assign the function's result directly to a variable.
FUNCTION AddNumbers(Num1 : INTEGER, Num2 : INTEGER) RETURNS INTEGER
DECLARE Total : INTEGER
Total ← Num1 + Num2
RETURN Total
ENDFUNCTION
// The Main Program using the function:
DECLARE Answer : INTEGER
Answer ← AddNumbers(10, 5)
OUTPUT Answer // Outputs 154. Parameters: BYVAL vs BYREF
When passing data (parameters) into a subroutine, you have to decide if the subroutine is allowed to permanently change the original variable.
A-Level Exam Tip 🎓
If an exam question asks you to update an array or a record using a procedure, you must pass it BYREF, otherwise the main program won't see any of your updates!
Common Syllabus Mistakes
Examiners routinely deduct marks for these subroutine errors:
❌ Calling a Function like a Procedure: You cannot write CALL AddNumbers(10, 5). Because a function returns a value, that value must be caught by a variable: Answer ← AddNumbers(10, 5).
❌ Forgetting the RETURN keyword: If you declare a FUNCTION, you must end it with a RETURN [Variable] statement before the ENDFUNCTION keyword. Our PseudoStudio compiler will throw an error if you forget this!
Interactive Exam Practice
Scenario: Write a FUNCTION named GetDouble that takes a single INTEGER parameter by value, multiplies it by 2, and returns the result.
Hover or tap the black box to reveal the examiner's solution. Click Run Code to test it instantly!
FUNCTION GetDouble(Num : INTEGER) RETURNS INTEGER DECLARE Result : INTEGER Result ← Num * 2 RETURN Result ENDFUNCTION // Test the function in the main program OUTPUT "Double of 5 is: ", GetDouble(5)