As programs grow, writing everything in one long block becomes unmanageable. Sub-programs let you package code under a name and reuse it. This lesson covers procedures, functions, parameters and why modular code is better.

What is a sub-program?

A sub-program is a named, self-contained block of code that performs one task. It is defined once and called from anywhere. The two main kinds are:

  • Procedure: performs actions but does not return a value.
  • Function: calculates and returns a value to the caller.

Procedures

PROCEDURE greet(name)
   output "Hello, " + name
END PROCEDURE

greet("Sam")        // Hello, Sam
greet("Priya")      // Hello, Priya

The call greet("Sam") sends the argument “Sam” into the parameter name. The terms are often confused: parameters appear in the definition, arguments are the actual values passed in a call.

Functions

FUNCTION area(width, height)
   return width * height
END FUNCTION

x = area(4, 5)      // x = 20
output area(2, 3)   // 6

A function’s returned value can be stored, printed or used in an expression. When return runs, the function ends immediately.

Parameters and scope

Variables created inside a sub-program are local: they exist only while it runs and are invisible outside. This prevents sub-programs from accidentally changing each other’s variables. Information goes in through parameters and out through return values.

Parameters can be passed by:

  • Value: the sub-program gets a copy, so changes do not affect the original.
  • Reference: the sub-program works on the original, so changes persist. (Arrays and objects are often passed this way.)

Why use sub-programs?

  • Reuse: write once, use many times.
  • Readability: meaningful names make the main program easy to follow.
  • Decomposition: a large problem splits into smaller tasks.
  • Testing: each sub-program can be tested separately.
  • Teamwork: different people can write different parts.
  • Maintenance: fix a bug in one place.

Recursion (HL preview)

A sub-program can call itself. A recursive function needs a base case to stop and a step that moves towards it:

FUNCTION factorial(n)
   if n = 0 then
      return 1
   else
      return n * factorial(n - 1)
   end if
END FUNCTION

factorial(3) evaluates 3 x 2 x 1 x 1 = 6. Without a base case it would never end.

Check your understanding

What distinguishes a function from a procedure?

  • A function returns a value
  • A function has no name
  • A procedure always returns a value
  • Functions cannot have parameters

Answer: A function returns a value. Procedures do not.

In area(4, 5), what are 4 and 5 called?

  • Arguments
  • Constants only
  • Loops
  • Arrays

Answer: Arguments. They are the values passed into the parameters.

What is a local variable?

  • A variable that exists only within the sub-program where it is declared
  • A variable available everywhere
  • A variable that never changes
  • A type of array

Answer: A variable that exists only within the sub-program where it is declared. This avoids accidental interference.

What does a recursive function need to avoid running forever?

  • A base case
  • More parameters
  • A loop
  • A global variable

Answer: A base case. It stops the recursion.

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