Learning FORTRAN In the Modern Era

Question

I\'ve recently come to maintain a large amount of scientific calculation-intensive FORTRAN code. I\'m having difficulties getting a handle on all of the, say, nuances, of a

Answer 1

Here's another one that has bit me from time to time. When you are working on FORTRAN code make sure you skip all six initial columns. Every once and a while, I'll only get the code indented five spaces and nothing works. At first glance everything seems okay and then I finally realize that all the lines are starting in column 6 instead of column 7.

For anyone not familiar with FORTRAN, the first 5 columns are for line numbers (=labels), the 6th column is for a continuation character in case you have a line longer than 80 characters (just put something here and the compiler knows that this line is actually part of the one before it) and code always starts in column 7.

Answer 2

As someone with experience in both FORTRAN (77 flavor although it has been a while since I used it seriously) and C/C++ the item to watch out for that immediately jumps to mind are arrays. FORTRAN arrays start with an index of 1 instead of 0 as they do in C/C++/Java. Also, memory arrangement is reversed. So incrementing the first index gives you sequential memory locations.

My wife still uses FORTRAN regularly and has some C++ code she needs to work with now that I'm about to start helping her with. As issues come up during her conversion I'll try to point them out. Maybe they will help.

Answer 3

Well, in one sense, you're lucky, 'cause Fortran doesn't have much in the way of subtle flow-of-control constructs or inheritance or the like. On the other, it's got some truly amazing gotchas, like the arithmetically-calculated branch-to-numeric-label stuff, the implicitly-typed variables which don't require declaration, the lack of true keywords.

I don't know about the "performance enhancing improvements". I'd guess most of them are probably ineffective, as a couple of decades of compiler technology have made most hinting unnecessary. Unfortunately, you'll probably have to leave things the way they are, unless you're planning to do a massive rewrite.

Anyway, the core scientific calculation code should be fairly readable. Any programming language using infix arithmetic would be good preparation for reading Fortran's arithmetic and assignment code.

Answer 4

I have used Fortran starting with the '66 version since 1967 (on an IBM 7090 with 32k words of memory). I then used PL/1 for some time, but later went back to Fortran 95 because it is ideally suited for the matrix/complex-number problems we have. I would like to add to the considerations that much of the convoluted structure of old codes is simply due to the small amount of memory available, forcing such thing like reusing a few lines of code via computed or assigned GOTOs. Another problem is optimization by defining auxiliary variables for every repeated subexpression - compilers simply did not optimize for that. In addition, it was not allowed to write DO i=1,n+1; you had to write n1=n+1; DO i=1,n1. In consequence old codes are overwhelmed with superfluous variables. When I rewrote a code in Fortran 95, only 10% of the variables survived. If you want to make the code more legible, I highly recommend looking for variables that can easily be eliminated.

Another thing I might mention is that for many years complex arithmetic and multidimensional arrays were highly inefficient. That is why you often find code rewritten to do complex calculations using only real variables, and matrices addressed with a single linear index.