Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

[FORTRAN] Division by zero allowed?

  1. Oct 29, 2012 #1
    Hi, I have an issue where I am squaring a very small complex number 1e-20 (FALFA2) which seems to make the value zero (both the real and imag part) and then dividing by it, the problem line is (all the numbers are complex):
    GALFA = ( FBETA * FAA / FALFA2 ) - ( FAB / FALFA )

    I get the error 'unknown floating point exception'. However, when another compiler compiled this code the executable works fine and I don't get an error for this (this is an old code that I am making some edits to now). Is there a way to just let GALFA=INF or something and continue, or do whatever the other compiler did (whatever that is)? Thank you!
  2. jcsd
  3. Oct 29, 2012 #2
    Do the calculation in double precision.
  4. Oct 29, 2012 #3
    How exactly do I do that?
  5. Oct 29, 2012 #4
    double precision or quadruple!

    Different compilers do behave slightly different, by default, and there may be a way to control their behavior, too...you may want to read the user's manual for your compiler. I know that with g95, for example, one can set an environment variable to let the program continue or terminate on a "divide by zero" error.

    I seem to recall seeing IEEE related function calls to control reporting this problems and behavior.

    You can turn a blind eye and let the program continue, but what's the point? are you going to believe the results on a run like that?.


    ...fix the program and guard against things like this...what is the order of magnitude of your numbers? when is a zero, zero?

    Can you scale your problem and work with larger numbers?
  6. Oct 29, 2012 #5
    tell which compiler you are using

    post the commands you are using to compile the program
  7. Oct 29, 2012 #6
    Hi Gsal,

    I am using silverfrost (ftn95), and I am not really using any commands I just click 'compile'. And yes, I would just like to turn a blind eye. As I said, I have the executable that is the exact same code and it works fine, I just need to add a few write/print statements, but due to this issue I get the error I described.
  8. Oct 30, 2012 #7
    Oh, boy....that is dangerous.

    First you need a bit of help and you need to be willing to be a bit more diligent about this...nothing good can come out of turning a blind eye...maybe this task is in the wrong hands.

    Second, I don't use Silverfrost, so, I cannot give details. Hopefully somebody will come along and give you step-by-step instructions on how to find out what the compilation command being used is and how to tweak it...OR, you could investigate yourself.

    Good luck.
  9. Oct 30, 2012 #8
    Check your FORTRAN manual.
  10. Oct 30, 2012 #9


    User Avatar
    Homework Helper

    My guess is that the older compiler uses higher precision floating point math, ending up dividing by a small number instead of zero. A similar thing happened with microsoft c. The old 16 bit compilers supported "long double" using the 80 bit format, but the 32/64 bit compilers changed this so that "long double" is the same as "double" at 64 bit format (perhaps an issue related to a win32 console mode program, but assembly code can still use the 80 bit format).
    Last edited: Oct 30, 2012
  11. Oct 30, 2012 #10


    User Avatar
    Science Advisor

    Just guessing here, but FALFA2 = FALFA*FALFA. Is that right? In that case you'd be much better off rearranging the calculation as :

  12. Oct 30, 2012 #11
    I was able to get around this by using a different compiler (g95), thank you everyone for the help!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook