FUNCTION

Statement: The initial statement of a function subprogram. A function subprogram is invoked in an expression and returns a single value (a function result) that is used to evaluate the expression.

Syntax

[prefix] FUNCTION name [([d-arg-list])] [suffix]
     [specification-part]
     [execution-part]
[CONTAINS
     internal-subprogram-part]
END [FUNCTION [name]]

prefix
(Optional) Is one of the following:

type [keyword]
keyword [type]

type
Is a data type specifier.

keyword
Is one of the following:

Keyword  Meaning 
RECURSIVE Permits direct recursion to occur. If a function is directly recursive and array valued, RESULT must also be specified.
PURE Asserts that the procedure has no side effects.
ELEMENTAL Restricted form of pure procedure that acts on one array element at a time.


name
Is the name of the function. If RESULT is specified, the function name must not appear in any specification statement in the scoping unit of the function subprogram.

The function name can be followed by the length of the data type. The length is specified by an asterisk (*) followed by any unsigned, nonzero integer that is a valid length for the function's type. For example, REAL FUNCTION LGFUNC*8 (Y, Z) specifies the function result as REAL(8) (or REAL*8).

This optional length specification is not permitted if the length has already been specified following the keyword CHARACTER.

d-arg-list
(Optional) Is a list of one or more dummy arguments. If there are no dummy arguments and no RESULT variable, the parentheses can be omitted. For example, the following is valid: 

  FUNCTION F

suffix
(Optional) Takes one of the following forms:

[RESULT (r-name)] lang-binding
lang-binding [RESULT (r-name)]

r-name
(Optional) Is the name of the function result. This name must not be the same as the function name.

lang-binding
Takes the following form:

BIND (C [, NAME=ext-name])

ext-name
Is a character scalar initialization expression that can be used to construct the external name.

specification-part
Is one or more specification statements, except for the following:

An automatic object must not appear in a specification statement. If a SAVE statement is specified, it has no effect.

execution-part
Is one or more executable constructs or statements, except for ENTRY or RETURN statements.

internal-subprogram-part
Is one or more internal subprograms (defining internal procedures). The internal-subprogram-part is preceded by a CONTAINS statement.

Description

The type and kind parameters (if any) of the function's result can be defined in the FUNCTION statement or in a type declaration statement within the function subprogram, but not both. If no type is specified, the type is determined by implicit typing rules in effect for the function subprogram.

Execution begins with the first executable construct or statement following the FUNCTION statement. Control returns to the calling program unit once the END statement (or a RETURN statement) is executed.

If you specify CHARACTER*(*), the function assumes the length declared for it in the program unit that invokes it. This type of character function can have different lengths when it is invoked by different program units; it is an obsolescent feature in Fortran 95.

If the length is specified as an integer constant, the value must agree with the length of the function specified in the program unit that invokes the function. If no length is specified, a length of 1 is assumed.

If the function is array-valued or a pointer, the declarations within the function must state these attributes for the function result name. The specification of the function result attributes, dummy argument attributes, and the information in the procedure heading collectively define the interface of the function.

The value of the result variable is returned by the function when it completes execution. Certain rules apply depending on whether the result is a pointer, as follows:

A function subprogram cannot contain a SUBROUTINE statement, a BLOCK DATA statement, a PROGRAM statement, or another FUNCTION statement. ENTRY statements can be included to provide multiple entry points to the subprogram.

You can use a CALL statement to invoke a function as long as the function is not one of the following types:

See Also

ENTRY, SUBROUTINE, PURE, ELEMENTAL, RESULT keyword, Function References, Program Units and Procedures, General Rules for Function and Subroutine Subprograms

Examples

The following example uses the Newton-Raphson iteration method (F(X) = cosh(X) + cos(X) - A = 0) to get the root of the function:

  FUNCTION ROOT(A)
    X  = 1.0
    DO
      EX = EXP(X)
      EMINX = 1./EX
      ROOT  = X - ((EX+EMINX)*.5+COS(X)-A)/((EX-EMINX)*.5-SIN(X))
      IF (ABS((X-ROOT)/ROOT) .LT. 1E-6) RETURN
      X  = ROOT
    END DO
  END

In the preceding example, the following formula is calculated repeatedly until the difference between Xi and Xi+1 is less than 1.0E-6:

The following example shows an assumed-length character function:

  CHARACTER*(*) FUNCTION REDO(CARG)
    CHARACTER*1 CARG
    DO I=1,LEN(REDO)
      REDO(I:I) = CARG
    END DO
    RETURN
  END FUNCTION

This function returns the value of its argument, repeated to fill the length of the function.

Within any given program unit, all references to an assumed-length character function must have the same length. In the following example, the REDO function has a length of 1000:

  CHARACTER*1000 REDO, MANYAS, MANYZS
  MANYAS = REDO('A')
  MANYZS = REDO('Z')

Another program unit within the executable program can specify a different length. For example, the following REDO function has a length of 2:

  CHARACTER HOLD*6, REDO*2
  HOLD = REDO('A')//REDO('B')//REDO('C')

The following example shows a dynamic array-valued function:

  FUNCTION SUB (N)
    REAL, DIMENSION(N) :: SUB
    ...
  END FUNCTION

The following shows another example:

      INTEGER Divby2
10   PRINT *, 'Enter a number'
      READ *, i
      Print *, Divby2(i)
      GOTO 10
      END
C
C     This is the function definition
C
      INTEGER FUNCTION Divby2 (num)
      Divby2=num / 2
      END FUNCTION

The following example shows an allocatable function with allocatable arguments:

MODULE AP
CONTAINS

 FUNCTION ADD_VEC(P1,P2)
  ! Function to add two allocatable arrays of possibly differing lengths.
  ! The arrays may be thought of as polynomials (coefficients)
   REAL, ALLOCATABLE :: ADD_VEC(:), P1(:), P2(:)

  ! This function returns an allocatable array whose length is set to
  ! the length of the larger input array.
  ALLOCATE(ADD_VEC(MAX(SIZE(P1), SIZE(P2))))
  M = MIN(SIZE(P1), SIZE(P2))
  ! Add up to the shorter input array size
  ADD_VEC(:M) = P1(:M) + P2(:M)
  ! Use the larger input array elements afterwards (from P1 or P2)
  IF(SIZE(P1) > M) THEN
    ADD_VEC(M+1:) = P1(M+1:)
  ELSE IF(SIZE(P2) > M) THEN
    ADD_VEC(M+1:) = P2(M+1:)
  ENDIF
 END FUNCTION
END MODULE

PROGRAM TEST
 USE AP
 REAL, ALLOCATABLE :: P(:), Q(:), R(:), S(:)
 ALLOCATE(P(3))
 ALLOCATE(Q(2))
 ALLOCATE(R(3))
 ALLOCATE(S(3))

 ! Notice that P and Q differ in length
 P = (/4,2,1/)  ! P = X**2 + 2X + 4
 Q = (/-1,1/)   ! Q =         X - 1
 PRINT *,' Result should be:    3.000000       3.000000       1.000000'
 PRINT *,' Coefficients are: ', ADD_VEC(P, Q)  ! X**2 + 3X + 3

 P = (/1,1,1/)  ! P =  X**2 +  X + 1
 R = (/2,2,2/)  ! R = 2X**2 + 2X + 2
 S = (/3,3,3/)  ! S = 3X**2 + 3X + 3
 PRINT *,' Result should be:    6.000000       6.000000       6.000000'
 PRINT *,' Coefficients are: ', ADD_VEC(ADD_VEC(P,R), S)
END