3.12 Calling Procedures

Calling a procedure

A procedure call (also called invoking a procedure) executes the statements inside that procedure. When a program calls a procedure, control transfers to the procedure's code, runs it, then returns to where it was called. Procedures can take parameters — inputs passed in when called — and may return a value using the parameter's actual arguments.

Parameters vs. arguments

A parameter is the placeholder variable listed in a procedure's definition. An argument is the actual value passed in when the procedure is called. For example, in `MOVE(distance)`, `distance` is the parameter; calling `MOVE(5)` passes the argument `5`. Multiple procedure calls can use the same procedure with different arguments to produce different results.

Why call procedures?

Calling existing procedures (built-in or programmer-defined) lets programmers reuse code without rewriting it, reducing complexity and errors. Procedure calls let a single block of abstracted code run multiple times with different arguments, supporting abstraction: the caller only needs to know what the procedure does and what inputs/outputs it uses, not how it works internally.

Key terms

Procedure call

An instruction that executes the statements inside a named procedure, then returns control to the calling code.

Parameter

A placeholder variable listed in a procedure's definition that receives an argument's value.

Argument

The actual value passed into a procedure when it is called.

Code reuse

Using existing procedures multiple times instead of rewriting code, reducing errors and complexity.

Benefit of abstraction in calls

The caller only needs to know what a procedure does and its inputs/outputs, not how it works internally.

Try an AP-style question

PROCEDURE double(n)
{
   RETURN(n * 2)
}
x ← double(double(3))

What is x?

1. 3
2. 6
3. 12
4. 9