FOR4_2.html

Subroutines are similar to, but more versatile than, external functions. Subroutines can be defined internally (following a CONTAINS statement) or externally (after the END PROGRAM statement). Functions are essentially simplified subroutines. The major differences between functions and subroutines are as follows:

Functions take input and return a single number, character string, logical result, or array to the program that referenced it. Subroutines can return a large amount of output or no data (i.e. they can just perform a task such as printing results or displaying output) to the referencing program.
The name of a function is set to the returning value. A subroutine name does not contain a value. Output from a subroutine is returned via the arguments contained in the output list (meaning if you want to return three values you must have one argument in the argument list for each output value).
Functions are referenced in the main program structure by using their name in an expression. Subroutines are referenced using a CALL statement.

The basic format of a subroutine description is as follows:

    SUBROUTINE name(dummy argument-list)
    specification part
    execution part
    END SUBROUTINE

The formal argument-list is a list of identifiers for the input and output to the subroutine. These values are called dummy because they are used to pass information between the subroutine and the main program.

The actual argument-list contains the variables, constants, or expressions that are to be used by the subroutine. The variables can be returned to the referencing program with values determined by the subroutine, or they can be used as input to the subroutine's execution. The arguments contained in the actual argument-list and the arguments contained in the dummy argument-list do not have to be the same; however, there does have to be the same number of arguments and their types must match.

Here is an example of a program calling a subroutine. The subroutine receives the name of a file, opens it, reads data, and returns that data to the referencing program.

    ! Calling the subroutine.
    ! The arguments do not need to be the same as
    ! the dummy arguments in the subroutine.
    ! values for vel, dens, and conc will be returned.

    CHARACTER(20), INTENT(IN) :: name
    REAL, INTENT (OUT) :: a, b, c
    INTEGER :: OpenStatus

    OPEN (UNIT = 10, FILE = name, ACTION = "READ", &
                IOSTAT = Openstatus)
    IF (OpenStatus /= 0) STOP "error opening file"