Functions and Subroutines

1
Functions and Subroutines

• Covers Chapters 6 and 7. We will start with 7
  first.
• A subroutine is a mini or sub program within your
  own program.
• Subroutines are called on to perform a specific
  function
• Why use subroutines?

2
Functions and Subroutines

• Suppose you had to print a complex header on a
  page over and over again in several different
  places...how would you do it.
• Suppose you had a complex arithmetic equation
  that you wanted to use in several different
  places on different variables.
• Suppose a problem was so big you could not solve
  it in its whole.

3
Functions and Subroutines

• You call a subroutine, when it is done, control
  returns to the statement after the call.
• Example
• XX1
• Call SQUARE(X)
• Print ,X

4
Format

• Format of a call statement to a subroutine
• CALL ROUTINENAME(ARGS)
• Format of the subroutine itself
• SUBROUTINE ROUTINENAME(ARGS)

5
Placement

• Where do they go?
• After the END statement in your program.

Main Program
Subroutines
6
Example

• PROGRAM XYZ
• INTEGER X
• CALL SQUARE(X)
• .
• .
• END
• SUBROUTINE SQUARE(Y)
• INTEGER Y
• YYY
• END

7
Functions

• Functions are also sub programs but return a
  single value.
• Functions are not called but placed in a regular
  Fortran Statement
• The Function is place where a value of its same
  type is expected.
• Functions cannot have values assigned to them
  only return values.

8
Functions

• type FUNCTION name(arguments)
• .
• .
• .
• RETURN
• END
• Note Somewhere in the subroutine a value must be
  assigned to the name of the subroutine.

9
Example

• REAL FUNCTION AVE(X,Y,Z)
• READ X,Y,Z
• AVE(XYZ)/3.0
• RETURN
• END
• EXAMPLE OF HOW IT IS USED
• AVETESTAVE(TEST1,TEST2,TEST3)

10
FUNCTIONS

• There must be at least one RETURN statement
• The type and word FUNCTION must proceed the name
  when writing the function.
• A value matching the type of the function must be
  assigned to the name at least once in the
  function.
• A return statement must exist in the function.