FORTRAN 90 vs C++ - an
educational perspective
- by John
Prentice
Below is a note I just sent to the chairman of a physics
department which requires freshmen to take a programming class
offered by the engineering department. That course has
traditionally taught Fortran77, but there is now a push on from the
engineering school to switch to teaching C++. As usual, the
arguments being mustered revolve around criticisms of Fortran 77 by
people who are too out of touch to know much of anything about
Fortran 90. In either case, I was asked by one of the senior
physics faculty to contribute my two cents worth about what
language they should be teaching. The appended note is the one I
sent and it summarizes our corporate experience with C++ and some
of our feelings about Fortran 90. Others may find this
interesting.
================= my note to the physics department
It was mentioned to me that the Engineering school is
considering changing Eng 120 to teach C++ instead of Fortran and
that you were soliciting comments about this with regard to physics
students. Even though I am not associated with the university, I
wanted to contribute my thoughts, which come from a commercial as
well as a research perspective.
As you know Quetzal Computational Associates specializes in
computational science. We currently have projects in computational
physics, earth sciences, and agriculture for clients which include
DoE and DoD laboratories as well as commercial clients as diverse
as hazardous waste companies and grain companies. During the last
year, we have developed numerical methods and codes based on them
for modeling contaminant flow in porous media, modeling the
structural mechanics of resonant sonic drilling rigs, modeling
bistatic ground penetration radar propagation in partially
saturated soils, modeling solid dynamics at high strain rates,
modeling the phenological development of corn and soy beans,
modeling solar insolation in the photosynthetically active spectrum
based on first principle atmospheric physics, and developed neural
networks for the detection of dust clouds from satellite
imagery.
One of our largest computational physics projects is the
development of advanced methods for modelling solid dynamics based
on first principle physics. This is a multi-year, multi-million
dollar project. This code numerically solves the partial
differential equations for continuum solid dynamics using a hybrid
finite volume/finite element technique coupled to advanced
equations of state and constitutive models for the solids and
fluids in the calculation. This code and others we work with are
huge number crunching codes, a modest 3d simulation will take 100
or more hours of Cray C-90 time to complete a single calculation.
By any standard, they are amongst the largest computational physics
simulations being done anywhere in the world. In addition, we are
on the forefront in the application of parallel computing to these
problems. We have projects to develop parallel versions of our
codes for a diverse collection of computers, including networks of
UNIX workstations, the IBM SP-2, the Cray T3D and the Intel
Paragon.
For all these projects, we employ Fortran 90 as our main
language. We do some development work in Fortran 77, C and C++,
but we are moving away from those languages as quickly as possible.
There are many reasons for our choice of Fortran 90, but first let
me say a bit about why we are not enthusiastic about C++. The
biggest strength of C++ is probably the availability of relatively
inexpensive and high quality C++ compilers for PCs. But that is a
pretty minor consideration in our business and it is outweighed by
the enormous liabilities we have observed with C++. First we
regard C++ as the weakest of the object oriented languages.
Objective C is a far more solid and well designed OOPS language,
C++ is really some OOPS capability slapped on top of C. C++ is
consequently extremely inefficient, inconsistent, overly large, and
enormously difficult to program in. The experience of our clients
mirrors that of our own, and in fact many DoE and DoD laboratories
are finding that their headlong rush to C++ has been a hideously
expensive mistake. I know of several C++ scientific coding
projects in the DoE that consumed millions of dollars and tens of
man-years, only to be abandoned because the resulting code was
enormously inefficient on both traditional serial computers and on
their large parallel supercomputers. Similar horror stories abound
throughout the programming community at this point. Bill Gates
claimed that his biggest mistake in designing their new NT
operating system was adopting C++ for the graphics coding, the
resulting code took years longer to write than it should have and
ran terribly slow. While OOPS is a solid development in the
computer science community, I think it is fair to say that C++ is
destined to be a passing fad, much like Pascal and Ada before
it.
The main reason C++ has attracted the attention it has in the
scientific community is because Fortran 77 was a terribly outdated
language. The many weaknesses of Fortran 77 were solved with
Fortran 90 however. Fortran 90 has every feature in C that is
important to scientific programming and most of the features of an
object oriented language (it lacks only inheritance and that is
likely going to be added in Fortran 2000). However unlike C and
C++, Fortran 90 is designed to generate executable codes that are
highly optimized and thus run extremely fast. An example is
pointers. Pointers are integral to C and C++ programming and
because the compiler cannot determine whether a pointer is aliased,
it is impossible for it to determine interprocedural dependencies.
The result is a significant degradation in optimization and
extremely slow execution speeds (for most scientific codes, C and
C++ generally produce code which is commonly an order of magnitude
slower than Fortran 90 codes, based on the benchmarks we and others
have done). Fortran 90 pointers are designed to give the
functionality of pointers, but with restrictions that eliminate
issues such as aliasing. From a programming perspective however,
an even more important point is that Fortran 90 has more natural
ways of expressing the functionality that C and C++ require
pointers to express. Because of this, Fortran 90 is a more natural
language to program in and the time required for debugging codes is
a fraction of that required by C and C++ (C++ is much worse that C, provided you are really employing an OOPS paradigm, since you find
yourself spending a lot of debug time going up and down inheritance
trees). Another important point is that the time required to learn
Fortran 90 is much less than the time to learn either C or C++.
Fortran 90 has another major advantage over C or C++. Modern
scientific computing, and computing in general, is moving toward
the use of parallel computers. Even PCs and workstations now come
with multiple processors, so parallelism is something that everyone
from an accountant to a physicist is encountering now. A major
problem in programming parallel computers however is the linear
memory model that is inherent to all procedural programming
languages, with the singular exception of Fortran 90. A linear
memory model is one that assumes that consecutive elements of an
array are consecutive in memory. This was a reasonable assumption
on traditional computers, but it is completely incorrect on a
parallel computer. Only Fortran 90 has addressed this problem and
providing standardized language support for parallelism. This
support includes array syntax and many intrinsics for doing array
operations varying from reduction operations such as array sums to
matrix operations. With the use of Fortran 90 operator overloading
and polymorphism, one can significantly extend the number of
operations that avoid any reliance on the linear memory model. The
fact that Fortran 90 moved away from a linear memory model is the
main reason that it has become the base for so many data parallel
languages such as Vienna Fortran, Fortran D, CRAFT, and High
Performance Fortran. The availability of data parallel dialects of
Fortran 90 is an especially large factor in favor of Fortran 90.
Compilers of High Performance Fortran, for example, are now coming
on the market for virtually every machine out there (including
networks of workstations) and writing parallel codes in this
language is straightforward. Of particular importance is that
porting a Fortran 90 code to High Performance Fortran is extremely
straightforward and codes written in High Performance Fortran can
be run unaltered on a Fortran 90 compiler (with the exception of
one HPF construct, the forall, which is being put into Fortran
95).
My own opinion is that scientists today need to know more than
one language or computing paradigm. And I think it is entirely
reasonable that students learn C++ before they graduate, though
even more important is that they learn how to program MATLAB and a
computer algebra system such as Maple or Macsyma. But the issue is
what freshmen should learn as their first language and for that I
would recommend Fortran 90 hands down. It is a better language for
scientific programming and is both easier to learn and use than the
alternatives. It is also much more likely to be the language
students will be employing in their jobs upon graduation and it is
the most promising route currently developing for the programming
of parallel computers.
Dr. John K Prentice, Quetzal Computational Associates, 3455 Main Ave.,
Suite 4, Durango, CO 81301-0201
Fax: 970-382-8981 E-mail:john@quetzalcoatl.com.
if you have comments please email: info@cts.com.au
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Updated 6 June 1996