Calling Procedures (College Board AP® Computer Science Principles): Revision Note

Procedure syntax and parameters

What is a procedure?

• A procedure is a named group of instructions that performs a specific task

• Procedures are also referred to as methods or functions in different programming languages

• Using procedures avoids rewriting the same code multiple times and makes programs easier to read and maintain

Calling a procedure

• A procedure call executes the instructions inside the procedure

• In AP CSP pseudocode, a procedure is called by writing its name followed by parentheses

PROCEDURE greet()
{
   DISPLAY("Hello!")
}

greet()
 

• Calling greet() executes the DISPLAY instruction and outputs "Hello!"

Parameters and arguments

• A parameter is a variable in the procedure definition that receives a value when the procedure is called

• An argument is the actual value passed into the procedure during the call

• Parameters allow the same procedure to work with different inputs each time it is called

• A procedure can also collect input by calling INPUT() inside its body instead of receiving values through parameters

PROCEDURE greet(name)
{
   DISPLAY(CONCAT("Hello, ", name))
}

greet("Alex")
greet("Sam")
 

• name is the parameter; "Alex" and "Sam" are the arguments

• The procedure displays a different greeting depending on the argument passed

Return values and control flow

What is a return value?

• A return value is a value that a procedure sends back to the part of the program that called it

• Not all procedures return a value; some only perform an action such as displaying output

• When a procedure returns a value, it can be stored in a variable or used directly in an expression

• The RETURN statement ends the procedure and sends a value back to the caller

• When RETURN is reached, the procedure stops executing immediately and no further statements in the procedure run

PROCEDURE square(n)
{
   RETURN(n * n)
}

result ← square(5)
DISPLAY(result)
 

• This stores 25 in result and displays it

Sequence interruption

• Calling a procedure temporarily interrupts sequential flow; the program jumps to the procedure body, executes it, then resumes at the point of the call

• The effect on the calling code depends on whether the procedure returns a value: