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

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.