Computer manufacturers have a natural tendency to compete with each other by providing Fortran systems which are "better" than before, usually by providing extensions to the language. This does not conflict with the Fortran Standard, provided that standard-conforming programs are still processed correctly. Indeed in the long term languages advance by the absorbtion of such extensions. In the short term, however, their use is more problematical, since they necessarily makes programs less portable.
When the latest Fortran Standard was issued in 1977 there was fairly widespread disappointment that it did not go just a little further in eliminating some of the tiresome restrictions that had persisted since the early days. The US Department of Defense issued a short list of extensions which manufacturers were encouraged to add to their Fortran77 systems. The most important of these were the following:
END DO statement
DO WHILE loop
INCLUDE statement
IMPLICIT NONE facility
One of the most irksome restrictions of Fortran77 is that symbolic names cannot be more than six characters long. This forces programmers to devise all manner of contractions, abbreviations, and acronyms in place of meaningful symbolic names. It is very tempting to take advantage of systems which relax this rule but this can have serious repercussions. Consider a program which makes use of variables called TEMPERATURE and TEMPERED. Many compilers will be quite happy with these, though a few will reject both names on grounds of length. Unfortunately there are also one or two compilers in existence which will simply ignore all letters after the sixth so that both names will be taken as references to the same variable, TEMPER. Such behaviour, while deplorable, is quite in accordance with the Standard which only requires systems to compile programs correctly if they conform to its rules.
The only way to be certain of avoiding problems like this is to ignore such temptations entirely and just use Standard Fortran. Many compilers provide a switch or option which can be set to cause all non-standard syntax to be flagged. Everything covered in this book is part of Standard Fortran unless clearly marked to the contrary.
Computer programming always requires a very high standard of care and accuracy if it is to be successful. This is even more vital when using Fortran than with some other languages, because, as explained above, the liberal rules of Fortran make it harder for the system to detect mistakes. To program successfully it is not enough just to conform to the rules of the language, it is also important to defend yourself against known pitfalls.
There is a useful lesson to be learned from the failure of one of the earliest
planetary probes launched by NASA. The cause of the failure was eventually traced
to a statement in its control software similar to this:
DO 15 I = 1.100
when what should have been written was:
DO 15 I = 1,100
but somehow a dot had replaced the comma. Because Fortran ignores spaces, this
was seen by the compiler as:
DO15I = 1.100
which is a perfectly valid assignment to a variable called DO15I and not at all what
was intended.
Fortran77 permits an additional comma to be inserted after the label in a DO
statement, so it could now be written as:
DO 15,I = 1,100
which has the great advantage that it is no longer as vulnerable to a single-point
failure.
There are many hazards of this sort in Fortran, but the risk of falling victim to them can be minimised by adopting the programming practices of more experienced users. To help you, various recommendations and guidelines are given throughout this book. Some of the most outdated and unsatisfactory features of Fortran are not described in the main part of the book at all but have been relegated to section 13.
There is not room in a book of this size to go further into the techniques of program design and software engineering. As far as possible everything recommended here is consistent with the methods of modular design and structured programming, but you should study these topics in more detail before embarking on any large-scale programming projects.
