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Old   May 4, 2004, 15:50
Default f77-g77
  #1
subhra
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I found that the f77 and g77 compilers values differing in precison from 8th digit..any idea why?
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Old   May 5, 2004, 04:46
Default Re: f77-g77
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Steve
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From your question, my assumptions are:

1) You are using Linux. 2) You have a choice between 2 compilers, one of which is a proprietary FORTRAN compiler (e.g. from Absoft). The other is the native g77.

If this is the case, you are bound to get different results, since g77 is a wrapper around f2c and as such, isn't really a FORTRAN compiler at all.

One thing you'll find when using f2c to convert/compile FORTRAN code is that the order of evaluation changes. This can affect precision. Also in expressions like:

IF(A .and. B)

expression B will always be evaluated with a FORTRAN compiler, but only if A is true if you use f2c.

- Steve
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Old   May 6, 2004, 13:28
Default Re: f77-g77
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jason
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Your information on g77 is old. It is a front-end to gcc:

http://gcc.gnu.org/onlinedocs/g77/Wh...-Fortran-.html

This does not mean that the f2c stuff is totally gone, but g77 is *not* just a wrapper around f2c.

jason
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Old   May 7, 2004, 18:27
Default f77-g77
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dattsub
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Thanks for comments to both of you...I had run 1000s of iterations of my research code using g77...Perhaphs, I could have done better with a proprietery compiler....but I will broach another apparantly significant observation in the next message..it's not only the compiler..
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Old   May 8, 2004, 13:29
Default Re: f77-g77
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jasond
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Since I did not originally say anything about your issue, I'll do that now. Different compilers do different things with the precision of constants - for example, if I want a double precision value for pi I could do something like:

pi=acos(-1.0)

This will probably give different results using different compilers (proprietary or not). It probably shouldn't, but it does (and I am not sure what the standard says about this). There are usually switches that control what exactly is done with the "1.0" as an argument (is it a double or a single?). G77 is somewhat vulenerable to this issue, but it is not the worst I have seen.

jason
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