Calling Procedures: Revision Notes (Computer Science Principles 3.12)

Recap

• A procedure is a named group of instructions; it may have parameters and return a value. (Method and function are other names.)
• Parameter: input variable in the definition. Argument: value given in the call, matched by position.
• A call interrupts sequential flow. After the last statement or a RETURN, control goes back to the point just after the call.
• RETURN(expression) sends the value back and ends the procedure at once.
• DISPLAY(expression) shows a value (and a space). It does not return anything.
• INPUT() returns a value typed by the user.
• A call that returns a value can be assigned or used inside an expression.

Key relationships

Idea	Notation	Example
Definition	PROCEDURE name(p1, p2) { ... }	PROCEDURE rectArea(width, height)
Call as statement	name(a1, a2)	greet("Mina")
Call as expression	result ← name(a1, a2)	a ← rectArea(3, 4)
Return a value	RETURN(expression)	RETURN(width * height)
Nested call	Inner call is evaluated first	DISPLAY(rectArea(rectArea(1, 2), 3))

Assumptions

• Arguments are evaluated before the procedure body runs.
• The number of arguments matches the number of parameters.
• A definition does nothing until the procedure is called.

Method for tracing a call

1. Write the parameters, then the argument values beside them.
2. Run the body line by line; note any DISPLAY output in order.
3. Stop at the first RETURN that runs.
4. Replace the call by its returned value and continue evaluating.

Mistakes to avoid

• Swapping the order of arguments.
• Confusing DISPLAY with RETURN.
• Running statements after a RETURN.
• Forgetting that DISPLAY inside a procedure happens during the call.
• Treating parameters as variables that existed before the call.

Worked mini-example

PROCEDURE f(n)
{
  DISPLAY(n)
  RETURN(n + 1)
}
DISPLAY(f(7))

Output: 7 8

The call f(7) runs first: it displays 7 and returns 8. Then the outer DISPLAY shows the returned 8. Output: 7 8.

Quick self-check

1. In PROCEDURE total(a, b) and the call total(2, 5), which are parameters and which are arguments?
2. What does a RETURN statement do besides giving a value back?
3. After x ← f(7) finishes, where does the program continue?

Answers

1. a and b are parameters; 2 and 5 are arguments.
2. It ends the procedure immediately.
3. At the statement immediately after the call.

Where marks are won and lost

Tracing questions give a procedure definition and a few calls and ask what is displayed or returned. Watch the order of arguments, early returns, and calls inside expressions. A second type asks which line of the program runs next after a call finishes. Others ask the difference between a parameter and an argument. Use the five-step checklist every time and track variables in a table. In the Create task, your own procedure must have at least one parameter that affects what it does, and you must show a call to it, so be ready to explain which argument went in and what came back.

Back to the full study guide for worked examples and explanations.