What are Sub-programs?
A sub-program is a named block of code that performs a specific task. Instead of writing the same code multiple times, you define it once and call it whenever needed. Sub-programs make programs easier to read, test, and maintain.
There are two types of sub-program in Edexcel 4CP0: procedures and functions.
Procedures
A procedure performs a task but does not return a value. It uses the keywords PROCEDURE / BEGIN PROCEDURE / END PROCEDURE.
PROCEDURE greet()
BEGIN PROCEDURE
SEND "Welcome to CSZone!" TO DISPLAY
SEND "Good luck with your revision." TO DISPLAY
END PROCEDURE
# Call the procedure
greet()
Procedure with Parameters
Parameters allow data to be passed into a sub-program so it can work with different values each time it is called.
PROCEDURE greetByName(name)
BEGIN PROCEDURE
SEND "Hello, " & name & "!" TO DISPLAY
END PROCEDURE
# Call with different arguments
greetByName("Alice") # outputs: Hello, Alice!
greetByName("Bob") # outputs: Hello, Bob!
Functions
A function performs a task AND returns a value using the RETURN keyword. It uses FUNCTION / BEGIN FUNCTION / END FUNCTION.
FUNCTION square(n)
BEGIN FUNCTION
RETURN n * n
END FUNCTION
// Call and use the returned value
SET result TO square(7)
SEND result TO DISPLAY // outputs 49
SEND square(3) TO DISPLAY // outputs 9
Function with Multiple Parameters
FUNCTION calculateArea(length, width)
BEGIN FUNCTION
RETURN length * width
END FUNCTION
SET area TO calculateArea(10, 5)
SEND "Area = " & area TO DISPLAY # outputs: Area = 50
Procedures vs Functions
| Feature | Procedure | Function |
|---|
| Returns a value? | No | Yes — uses RETURN |
| Syntax | PROCEDURE / BEGIN PROCEDURE / END PROCEDURE | FUNCTION / BEGIN FUNCTION / END FUNCTION |
| Called how | Called as a statement: greet() | Called in an expression: SET x TO square(4) |
| Best used for | Performing actions (display, write to file) | Calculating and returning a result |
Benefits of Sub-programs
- Reusability — write once, call many times; avoids duplicating code
- Readability — meaningful sub-program names make code easier to understand
- Easier testing — each sub-program can be tested independently
- Maintainability — fixing a bug in one place fixes it everywhere the sub-program is called
- Decomposition — naturally supports breaking a large problem into smaller parts
📝 Exam Tip: The key difference: a function RETURNS a value; a procedure does NOT. If a question asks for a value to be calculated and sent back to the calling code, always use a function with RETURN.
⚠️ Common Mistakes
- Using a procedure when a function is needed — if the sub-program needs to send a value back, use FUNCTION and RETURN
- Forgetting BEGIN PROCEDURE / BEGIN FUNCTION keywords — Edexcel pseudocode requires them
- Confusing parameters (in the definition) with arguments (the actual values passed when calling)