From: "Vyacheslav O. Myskin" Organization: BINP RAS, Novosibirsk, Russia To: kagel AT quasar DOT bloomberg DOT com Date: Fri, 7 Feb 1997 00:06:30 +0600 Subject: Re: double-->int: What's wrong here? Reply-To: V DOT O DOT Myskin AT inp DOT nsk DOT su Cc: djgpp AT delorie DOT com In-Reply-To: <9702061427.AA26252@quasar.bloomberg.com > References: (message from Eli Zaretskii on Thu, 6 Feb 1997 08:44:11 +0200 (IST)) Message-Id: <6CF0313A6D@csd.inp.nsk.su> Hello! > > The output is: > > n1=214 n2=215 d2=215.000000 > > > > So why n1 is not equal to n2? It's not fun because I used n1 as an > > argument to malloc() and kept getting SIGSEGVs later :( > > Never assume floating-point computations are exact: they aren't. The > exact reason for what your example printed are immaterial (it's a long > and quite dull story; you might consider adding -S to gcc command line > and looking at the assembly it generates to see what's going on). The > real lesson is that you should always round the FP numbers explicitly > before using them as counters of anything. In your case, say n1 = > .05/d1 + 0.5 and live happily ever after. > > But Eli, 1.0/4300.0 ==> 0.000232558139535 > and n1 = .05/d1 ==> 215.000000000000000 > to the limit of a double's precision (the next 5 digits would 86000 in the > 80bit internal x87 representation. There is no imprecision in this > calculation due to binary floating point effects and adding 0.5 will not > change the results. Vyacheslav may have indeed found something. > Vyacheslav are you using the emulator or an x87? Which? I ran it on 486DX, so it's x87. The assembler output ( gcc -S ...) shows that n1 is stored as an integer (fistp) immediately after calculation; n2 is converted after saving the value of d2 and reloading it (as it should be for unoptimized output). The ONLY difference is this fstp/fld combination. Is it OK that the data saved in memory is not identical to that in the FPU registers (can't assume anything else) ? Shame on me, my competence in FPU topics is equal to zero, hope haven't misspelled instruction names ;) Vyacheslav > > -- > Art S. Kagel, kagel AT quasar DOT bloomberg DOT com > > A proverb is no proverb to you 'till life has illustrated it. -- John > Keats