transfer from real to logical
in [Fortran]
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From: Dan Nagle on 7 Dec 2009 06:16 Hello, On 2009-12-07 03:13:50 -0500, Frank <frank(a)example.invalid> said: > Do C and Fortran have the same bit > model for a float/default real? Hardware encodes floating point. A program written in either C or Fortran, running on particular hardware, will use the supported encoding (or it will execute very slowly). The languages do not have encodings. The languages tell you how to access the facilities of IEEE 754, if available. The languages may tell you the effects of operations, but not how the operations are done. -- Cheers! Dan Nagle
From: Tim Prince on 7 Dec 2009 08:35 Frank wrote: > On Fri, 4 Dec 2009 17:16:19 -0500, Dan Nagle wrote: > >> >> Logicals have two values, true and false. >> These may be encoded at the processor's whim. >> >> I have seen zero v nonzero, positive v negative, >> even v odd, and probably something else I can't recall. >> >> Others have, no doubt, seen other encodings. >> >> That way are dragons. > > This shakes my faith a little bit. Do C and Fortran have the same bit > model for a float/default real? Neither C nor Fortran has a useful relationship between floating point formats and true/false bit patterns. If you had faith in such a thing, it was misplaced. Dan already answered what you asked explicitly. C guarantees that the relational operators return 0 or 1. A very few Fortran compilers match C in that respect. If you think about it, when that is done, it works under any of 3 possibilities Dan mentioned above.
From: frank on 7 Dec 2009 16:40 On Mon, 07 Dec 2009 05:35:49 -0800, Tim Prince wrote: >>> That way are dragons. >> >> This shakes my faith a little bit. Do C and Fortran have the same bit >> model for a float/default real? > Neither C nor Fortran has a useful relationship between floating point > formats and true/false bit patterns. If you had faith in such a thing, > it was misplaced. Dan already answered what you asked explicitly. C > guarantees that the relational operators return 0 or 1. A very few > Fortran compilers match C in that respect. If you think about it, when > that is done, it works under any of 3 possibilities Dan mentioned above. But one and zero are comparable in that one is greater than zero every day of the week. Furthermore, I think there's guarantees that 0 is false on the C side of it. I for one wouldn't understand a logic without that being the case, although I wouldn't doubt that some pathological alternative exists. I've been working up this same material in C, where they don't have an intrinsic like btest and rely on left and right shifts and a mask instead, and then there's the ultimate comparison to zero: printf("%d", (*p & mask) > 0); If we take a float and transfer it to an integer of an appropriate width, do I then have the bit model discussed at the beginning of 8.8 MR&C? Can I speak then of "setting a bit" and thinking that that means giving it the value one? -- frank "Guns: yes, they are harmful."
From: frank on 7 Dec 2009 16:50 On Mon, 07 Dec 2009 06:16:31 -0500, Dan Nagle wrote: > Hello, > > On 2009-12-07 03:13:50 -0500, Frank <frank(a)example.invalid> said: > >> Do C and Fortran have the same bit >> model for a float/default real? > > Hardware encodes floating point. Ok. > > A program written in either C or Fortran, running on particular > hardware, will use the supported encoding (or it will execute very > slowly). > > The languages do not have encodings. > The languages tell you how to access the facilities of IEEE 754, if > available. > > The languages may tell you the effects of operations, but not how the > operations are done. If I take a float in C, assign it a value, pass a pointer to it to fortran using the ISO_C_BINDING, dereference the pointer to assign its value to a default real, do I necessarily have the same value? -- frank "Guns: yes, they are harmful."
From: glen herrmannsfeldt on 7 Dec 2009 17:07
frank <frank(a)example.invalid> wrote: (snip) > If I take a float in C, assign it a value, pass a pointer to it to > fortran using the ISO_C_BINDING, dereference the pointer to assign its > value to a default real, do I necessarily have the same value? Theoretically you should use TYPE(C_FLOAT) to get the appropriate type, but I would be pretty surprised to find that different from default REAL. They might be different on hardware with more than one floating point type of the appropriate size. VAX has two different 64 bit floating point formats. With the addition of decimal float to the IEEE standard, it would be possible for a compiler to use that as its default floating point type. -- glen |