Subroutines: Functions and Procedures

What subroutines are

A subroutine is a named block of code that performs a specific task and can be called (run) whenever needed. Subroutines come in two kinds: functions and procedures. They make programs shorter, clearer and easier to maintain.

Why use subroutines?

• Avoid repetition — write the code once, call it many times.
• Break a big problem into smaller parts (decomposition) — each subroutine solves one task.
• Easier to read, test and debug — you can test each subroutine separately.
• Reusable — the same subroutine can be used in other programs.

Functions vs procedures

	Function	Procedure
Returns a value?	Yes – gives a result back	No – just performs actions
Typical use	Calculate something you use later	Do a task (e.g. print a menu)
Example call	total = add(3, 4)	showMenu()

function add(a, b)
   return a + b
endfunction

procedure greet(name)
   output "Hello " + name
endprocedure

Parameters and arguments

• A parameter is a variable listed in the subroutine definition — the data it needs to do its job (a and b above).
• An argument is the actual value you pass in when calling it (add(3, 4) passes 3 and 4).
• Passing data in through parameters makes a subroutine flexible — it works on different values each time.

Return values

A function sends a result back with return. The calling code can store or use it:

answer = add(10, 5)   // answer becomes 15

Local vs global variables

• A local variable is created inside a subroutine and only exists there — it can't be seen by the rest of the program.
• A global variable is declared outside all subroutines and can be used anywhere.
• Prefer local variables: they avoid accidental clashes, keep subroutines self-contained, and free memory when the subroutine ends.

Worked example

What does output add(add(2, 3), 4) print, using the add function above?

1. Inner call: add(2, 3) = 5.

2. Outer call: add(5, 4) = 9. ✓

Common mistakes

• Confusing a function (returns a value) with a procedure (doesn't).
• Mixing up parameter (in the definition) and argument (the value passed in).
• Trying to use a local variable outside its subroutine.

Exam tips

• Learn the one-line difference: function returns a value, procedure does not.
• Be ready to explain two benefits of subroutines (e.g. reuse + easier debugging).
• Know why local variables are preferred over global ones.

Key facts to remember

• A subroutine is a named, reusable block of code — a function (returns a value) or a procedure (performs a task).
• Parameters receive data (the passed-in values are arguments); functions send results back with return.
• Local variables exist only inside a subroutine; global ones exist everywhere — locals are safer.