17. Status of Fortran 95

Switch to the Swedish version Most of the new features are from HPF.

Fortran 95 was published as ISO/IEC 1539-1:1997 on December 15, 1997.

The preliminary revision has been published as "Special Issue, Fortran 95, Committee Draft, May 1995" in the Fortran Forum, Vol. 12, No. 2, June 1995.

Two important issues, exception handling, especially for floating point, and interoperability between languages (mixed language programming) are not addressed in Fortran 95. Exception handling is discussed in some papers by John Reid, see the proceedings of the Kyoto Workshop on Current Directions in Numerical Software and High Performance Computing.

17.1 New features

  1. The statement FORALL as an alternative to the DO-statement
  2. Partial nesting of FORALL and WHERE statements
  3. Masked ELSEWHERE
  4. Pure procedures
  5. Elemental procedures
  6. Pure procedures in specification expressions
  7. Revised MINLOC and MAXLOC
  8. Extensions to CEILING and FLOOR with the KIND keyword argument
  9. Pointer initialization
  10. Default initialization of derived type objects
  11. Increased compatibility with IEEE arithmetic
  12. A CPU_TIME intrinsic subroutine
  13. A function NULL to nullify a pointer
  14. Automatic deallocation of allocatable arrays at exit of scoping unit
  15. Comments in NAMELIST at input
  16. Minimal field at input
  17. Complete version of END INTERFACE
Pure functions are functions without side effects, and elemental functions are pure functions with only scalar arguments and with scalar result.

17.2 Deleted features

Five features are deleted from Fortran 90.
  1. real and double precision DO loop index variables
  2. branching to END IF from an outer block
  3. PAUSE statements
  4. ASSIGN statements and assigned GO TO statements and the use of an assigned integer as a FORMAT specification
  5. Hollerith editing in FORMAT
These are all from the list of obsolescent features of Fortran 90 (but not the complete list).

It is permitted for a compiler to have extensions to the standard, provided that these can be flagged by the system. It is very common for compilers to include the two features (see Appendix 4) that were removed from Fortran when Fortran 77 was introduced, and it is expected that this tradition will continue.

17.3 Obsolescent features

In the obsolescence list below those items that were not in the corresponding list of Fortran 90 are indicated New. All items in the list below are being considered for removal at the next revision, and should therefore be avoided.

The most probable candidates, in our opinion, for removal at the next revision (in the next century) are the first six.

  1. Arithmetic IF-statement
  2. Terminating several DO-loops on the same statement or terminating the DO-loop in some other way than with CONTINUE or END DO
  3. Alternate return
  4. Computed GO TO statement New
  5. Statement functions New
  6. DATA statements among executable statements New
  7. Assumed character length functions New
  8. Fixed form source code New
  9. CHARACTER* form of CHARACTER declaration New

To replace CHARACTER*LENGTH with CHARACTER(LEN=LENGTH) or CHARACTER(LENGTH) is simple, as are the other items above. For example, item 6 is just to move all DATA statements to the top of the program unit, before the executable statements. Statement functions are of course replaced with internal functions.

17.4 Schedule

The expected schedule for Fortran 95 is as follows.
  • April 1995: Technical completion
  • End of May 1995: Document available
  • July - September 1995: Review and ISO CD ballot
  • November 1995: Revision based on ballot comments
  • March 1996: Revised document available
  • 26 January 1997: End of the ISO DIS international ballot
  • 15 December, 1997: Fortran 95 published as ISO/IEC 1539-1:1997 (replacing Fortran 90, ISO/IEC 1539:1991).

17.5 Description of the new features

 An explanation of the new features follows.

  1. The statement FORALL

    The statement FORALL is introduced as an alternative to the DO-statement. The main difference is that while the execution order of the various parts of the DO-loop is very strict, the execution order of the FORALL is less strict, thus permitting parallel execution. For further information we refer to our HPF Appendix or directly to the HPFF home page.

  2. Partial nesting of FORALL and WHERE statements

    A FORALL statement can include a WHERE statement.

  3. Masked ELSEWHERE

    It is now permitted to mask not only the WHERE statement of the WHERE construct, but also its ELSEWHERE, which now may be repeated. 
    
	WHERE (condition_1)

	...

	ELSEWHERE (condition_2)

	...

	ELSEWHERE 

	...

	END WHERE

  4. Pure procedures

    Pure functions are functions without side effects. That a function has no side effects can be indicated with the new PURE prefix.

    A long list of constraints is given in the standard proposal, section 12.6.

    Pure subroutines are defined in a similar way. The only major difference is that "side effects" are of course permitted for arguments associated with dummy arguments specified INTENT(OUT) or INTENT(INOUT).

    The advantage with knowing that a function is pure is that this fact simplifies parallel execution.

  5. Elemental procedures

    Elemental functions are pure functions with only scalar dummy arguments (not pointers or procedures) and with scalar result (not pointer). That a function is elemental can be indicated with the new ELEMENTAL prefix. The RECURSIVE prefix my not be combined with the ELEMENTAL prefix. The PURE prefix is automatically implied by the ELEMENTAL prefix.

    An elemental function may be used with arrays as actual arguments. These must then be conformable. The result is the same array as if the function had been used individually with each of the array elements, in any order.

    Elemental subroutines are defined in a similar way. The only major difference is that "side effects" are of course permitted for arguments associated with dummy arguments specified INTENT(OUT) or INTENT(INOUT).

    The advantage with knowing that a function is elemental is that this fact simplifies parallel execution, even more than if it is only pure.

  6. Pure procedures in specification expressions

    Pure functions can be used in specification expressions if certain conditions are fulfilled, see the standard proposal, section 7.1.6.2.

    Specification expressions can be used to specify array bounds and character lengths of data objects in a subprogram.

  7. Revised MINLOC and MAXLOC

    The array location functions MINLOC and MAXLOC are extended with the optional argument DIM corresponding to those for the array functions MINVAL and MAXVAL.

  8. Extensions to CEILING and FLOOR

    The new numerical functions CEILING and FLOOR are extended with the KIND keyword argument, in the same way as for INT and NINT. The result is an integer, but of the specified KIND or subtype, not necessarily the standard (default) integer subtype.

  9. Pointer initialization

    The new function NULL can be used at specification to define a pointer to be initially disassociated, see below.

  10. Default initialization of derived type objects

    Means are now available to specify default initial values for derived type components. It is the usual way of using the equal sign (or pointer assignment) followed by the value (perhaps an array constructor). Initialization does not have to apply to all components of a certain derived type.

    A simple example. In Appendix 3, section 12, we introduced a sparse matrix.

    A numerically interesting example is a sparse matrix A with at most one hundred non-zero elements, which can be specified with the following statement, where now initialization is done to 2.0 for all elements. 

    
    TYPE NONZERO

           REAL :: VALUE = 2.0

           INTEGER :: ROW, COLUMN

    END TYPE
    and 
    
    TYPE (NONZERO)  :: A(100)
    You then get the value (which will be 2.0) of A(10) by writing A(10)%VALUE. The default value can be individually changed with an assignment, for example 
    
	A(15) = NONZERO(17.0,3,7)

  11. Increased compatibility with IEEE arithmetic

    The IEEE arithmetic has for floating point numbers one bit pattern for plus zero and another one for minus zero. Processors that distinguish between them shall treat them as identical
    • In all relational operations
    • As input arguments to all intrinsics except SIGN
    • As the scalar expression in the arithmetic IF-statement
    In order to distinguish between the two cases, the function SIGN has to be used. It is generalized so that the sign of the second argument is considered also if the value is a floating-point zero.

  12. A CPU_TIME intrinsic subroutine

    The subroutine CPU_TIME(TIME) belongs of course to intrinsic subroutines. In the scalar real variable TIME the present processor time is returned in seconds. If the processor is unable to provide a timing, a negative value is returned instead. As usual, the time for a certain computation is obtained by subtracting two different calls to the timing routine.

    The exact nature of the timing is implementation dependent, a parallel processor might reurn an array of times corresponding to the various processors. The difference between CPU-time and system time is also implementation dependent.

  13. A function NULL to nullify a pointer

    This function can be used at specification to define a pointer to be initially disassociated, in the example below the array VECTOR. 
    
	REAL, POINTER, DIMENSION(:) :: VECTOR => NULL()
    The argument is not necessary, if present it determines the characteristics of the pointer, if not present, the characteristics are determined from context.

    The function belongs to section 20, pointer inquiry functions, which is now renamed "Pointer association status functions".

  14. Automatic deallocation of allocatable arrays at exit of scoping unit

    If the user has not explicitly deallocated local allocatable arrays at the exit of the scoping unit, this deallocation is now performed automatically, thus decreasing the memory required.

  15. Comments in NAMELIST at input

    It is now permitted to use comments in the usual way with !

  16. Minimal field at output

    In order to obtain an optimized use of the available positions it is now possible to only give the number of decimals, if any, and not the total field width, at the FORMATs B, F, I, O, and Z. Examples are I0 and F0.6. The result is of course not a field with zero digits, but with a suitable number of digits.

  17. Complete version of END INTERFACE

    Also END INTERFACE can now be given in a complete variant, so the second interface in chapter 10 can be given either in the old version as 
    
	INTERFACE SWAP

		MODULE PROCEDURE SWAP_R, SWAP_I, SWAP_C

	END INTERFACE
    or in the new preferred way as 
    
	INTERFACE SWAP

		MODULE PROCEDURE SWAP_R, SWAP_I, SWAP_C

	END INTERFACE SWAP
    This can also include an optional generic specification.

Last modified: 11 September 1998
boein@nsc.liu.se