2.4.2 Fortran 90/95

Almost immediately after Fortran77 was standardised, work began on its successor. Although this project was named Fortran 8X, the work took long enough that the final standard was named Fortran 90.

The Fortran standard is maintained by ISO committee JTC1/SC22/WG5, and the current version of the standard is ISO/IEC 1539-1 : 1997.

Fortran 90 was an attempt to respond to the numerous developments in language design, seen since Fortran's last standardisation in the sixties and seventies. In contrast to Fortran 77, which aimed to standardise existing practice, Fortran 90 was an attempt to push forward the development of Fortran as a language. A summary of the new features (adapted from [metcalf96]) is:

• Free format source code form (no more column-counting).
• A means for the language to evolve by labelling some features as `obsolescent'.
• Array operations (for example, X(1:N)=R(1:N)*COS(A(1:N))).
• Pointers.
• Improved facilities for numerical computation including a set of numeric enquiry functions.
• User-defined derived data types composed of arbitrary data structures and operations upon those structures.
• Facilities for defining collections called `modules', useful for global data definitions and for procedure libraries. These support a safe method of encapsulating derived data types. Operator overloading and prototyping.
• Requirements on a compiler to detect the use of constructs that do not conform to the syntax of the language or are obsolescent.
• New control constructs such as the select case construct, an exit and a new form of the do.
• The ability to write internal prodedures and recursive procedures, and to call procedures with optional and keyword arguments.
• Dynamic storage (automatic arrays, allocatable arrays, and pointers).
• Improvements to the input-output facilities, including handling partial records and a standardized namelist facility.
• Many new intrinsic procedures, (date, precision, arrays, ...)

Fortran 90 is backwards compatible with Fortran 77, so that every strictly conformant Fortran 77 program is also a valid Fortran 90 program. However, many of Fortran 77's odder features are strongly deprecated, and may disappear in the next revision of Fortran, due to appear sometime in the next decade. Fortran 95 is a minor revision of Fortran 90.

There is an increasing number of books on Fortran 90/95; for a booklist, refer to the Fortran Market at the URL above. I am, of course, reluctant to recommend books I have not used myself, so I will simply note that I have heard good things about [metcalf96], and that the first author was prominent in the negotiations concerning the development of the Fortran 90 standard.

If you plan to use Fortran 90/95, but avoid the deprecated parts of Fortran 77 altogether, you might be interested in F, which is a subset of Fortran 90 with all the deprecated features removed. You can obtain an F compiler from Imagine1, including a free educational version for Linux.

Several Fortran compilers were reviewed in a short report of January 1997 by the high performance computing project at Liverpool.

It is occasionally necessary to produce code in a mixture of Fortran and C. See Section 2.5.4.