Procedures: Parameters

1
Procedures Parameters

• If using global variables is a bad idea, what do
  you use when main needs to share variables with
  other procedures?
• Answer Parameters

2
Parameters

• PROGRAM Example
• VAR A,B integer
• PROCEDURE Proc (Index1, Index2 integer)
• BEGIN
• END
• BEGIN main
• Proc (A,B)
• END.

3
Parameters (cont)

• Formal (dummy) parameters The parameter list in
  the PROCEDURE declaration.
• PROCEDURE Proc (Index1,Index2 integer)Index1
  and Index2 are the formal parameters
• Actual parameters The parameters contained in
  the activating statement.
• Proc (A,B)A and B are the actual parameters

4
Value parameters

• Value parameters should be thought of as one way
  parameters. Your activating statement sends the
  values to the procedure. Once the procedure
  finishes, the formal parameter value is lost.
  Therefore, if it changes, there will be no side
  effect to the main program.

5
Variable Parameters

• What if you want the value of the actual
  parameters to change after the procedure
  finishes?Answer Use Variable parameters

6
Variable parameters (cont)

• PROGRAM Example
• VAR A,B integer
• PROCEDURE Proc (VAR Index, Index2 integer)
• BEGIN
• END
• BEGIN main
• Proc (A,B)
• END.

7
How does the compiler deal with each type of
parameter?

• For every variable parameter, the formal and
  actual parameters share the same memory location.
  What is passed is a reference to a memory
  location.
• For every value parameter, the formal and the
  actual parameters are each given a separate
  memory location.

8
Parameters (cont)

• One procedure can contain both types of
  parameters
• PROCEDURE Sample (VAR A integer B
  char)
• You must rewrite VAR for every variable type if
  you want variable parameters
• PROCEDURE Sample2 (VAR A integer VAR
  Bchar)

9
Exit Statement

• What it you want a procedure to end before the
  last line of the procedure?
• You can do so by placing the statement EXIT
  anywhere in the procedure.
• Reasons why you need this statement will become
  more apparent in the next chapter.

10
A few final comments on procedures

• What should go into a procedure?
• Bite size chunks
• complete operations
• For now, a procedure can only call another
  procedure which appears before itself.
• Only use variable parameters when it is necessary.

11
Homework Due March 10, 2000

• It is homework 4 on Maratecks
  homepagehttp//cs.nyu.edu/courses/fall97/A22.00
  02/hw4.html