Subprograms

1
Subprograms

• A subprogram is a part of the program that has
  all the elements of the program itself.
• Subprograms facilitate the translation of the
  top-down algorithm to the final program because
  they allow us to concentrate on one task at a
  time.
• Subprograms are called or activated from the
  Main Program

2
Subprograms Procedures

• For now we will look at a type of subprogram
  called a procedure.
• A Pascal procedure is similar in construction to
  a program. It can contain its own VAR
  declaration and/or CONST definition, and it
  contains its own statement part which begins with
  BEGIN and ends with END

3
Procedures (cont)

• Differences between the construction of a program
  and a procedure
• The reserved word PROCEDURE replaces the reserved
  word PROGRAM.
• The final END is followed by a semicolon instead
  of a period.

4
Advantages of using procedures

• If you want to perform a task more than once in a
  program, you just reference the procedure each
  time. This avoids repeating the same code.
• The main program can be written as a sequence of
  tasks which makes it easier for humans to
  understand.

5
Global vs. Local variables

• Global identifiers are declared in the main
  program. These variables can be used in the main
  program as well as in the procedures that are
  called from the main program.
• Local identifiers are declared in the procedures.
  These variables can only be used by the
  procedure in which they are declared.

6
It is not a good idea to use global variables in
your procedures

• Doing so can cause side effects. A side effect
  occurs when you unintentionally assign a new
  value to a global variable within a procedure.
  When you return to the main program and use that
  variable, you will get an unexpected result.

7
What if we declare the same identifier in both
the main program and one of its procedures?

• The compiler will establish two different memory
  locations for the variables. It is as if they
  had different identifiers.
• One location is available in the main program.
• The other is available only in the
  procedure. ie. It takes precedence inside the
  procedure.

8
Homework 5

• Due November 2, 1999
• It is homework 4 on Maratecks
  homepagehttp//cs.nyu.edu/courses/fall97/A22.00
  02/hw4.html

9
Next class midterm review

• Please be prepared to ask me questions
• You go over and be comfortable with
• my old midterm
• Maratecks old midterm
• my program which converts chars to decimals using
  a FOR DO loop
• nested loops
• plus everything I listed last class