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 Some of the most outdated and unsatisfactory features of Fortran are not described in the main part of the book at all but have been relegated to section 13.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.