On Mon, 10 Aug 2020 at 10:08, wei zhao <zwleopard....@gmail.com> wrote: > > thanks Bill. > > I am confused. As you said, i can NOT initialize mpf_t via a double, right?
I did not say that. I said that doubles have 53 bits precision. You cannot get more precision from mpf if you start with 53 bits of precision at the input. > The reason I turn to multiple-precision is that I hope mpir could provide > more precision, double does have 53bit precision, but mpf_z should have more. It does allow more precision, but you cannot start with less information than you want to end up with. mpf is not able to add precision that wasn't there to start with. The information was already lost in the conversion of 0.1 decimal to a double (binary). This happened before mpf even got the value. > > if so, mpir is not more accurate than double, because it adds some random > numbers at the end, which will affect the computation result. No it does not "add random numbers". This is what 0.1 looks like to 53 bits of precision, which is precisely what you passed into the mpf function. It stored exactly what you gave it. > > any idea to solve this issue？ I already told you exactly how to solve the issue in my previous post. Did you try it? Decimal fractions aren't always exactly representable in (limited precision) binary. One way to solve it would be to put in an integer, then divide by a power of 10. Integers and powers of 10 are exactly representable in binary. Bill. > > > thanks. > > > > 在 2020年8月10日星期一 UTC+8下午2:45:56，Bill Hart写道： >> >> You are setting it to the double 0.1, which only has 53 bits >> precision. You can't get more precision than you put in. >> >> Try setting it to 0.5 first (which can be represented exactly in >> binary), and then divide by the integer 5, which is also able to be >> represented exactly in binary. Then you will get the 128 bits you are >> after. >> >> By the way, the mpf module isn't used much any more. You should look >> into mpfr (mpfr.org) instead, which is quite often much faster, and >> has more functionality and development effort. >> >> Bill. >> >> On Mon, 10 Aug 2020 at 08:32, wei zhao <zwleop...@gmail.com> wrote: >> > >> > hi all, >> > >> > I wrote a simple code to test mpir and found a strange thing. >> > >> > the code is >> > >> > mpf_t a; >> > mpf_init2(a, 128); >> > mpf_set_d(a, 0.1); >> > >> > mp_exp_t exp; >> > char str[256]; >> > mpf_get_str(str, &exp, 10, 128, a); >> > I checked str, and it was "1000000000000000055511151231257827021182" >> > instead of "1000000000000000000000000000000000000000". >> > >> > anybody knows why? Does not it affect the computation precision? how can >> > I make the variable more accurate? >> > >> > thansk a lot >> > >> > wei >> > >> > -- >> > You received this message because you are subscribed to the Google Groups >> > "mpir-devel" group. >> > To unsubscribe from this group and stop receiving emails from it, send an >> > email to mpir-...@googlegroups.com. >> > To view this discussion on the web visit >> > https://groups.google.com/d/msgid/mpir-devel/4dca16b4-7b7d-4a16-91c6-834def1f8f97o%40googlegroups.com. > > -- > You received this message because you are subscribed to the Google Groups > "mpir-devel" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to mpir-devel+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/mpir-devel/22b82e2c-cc4d-4557-9f8d-ce7c434aa5ado%40googlegroups.com. -- You received this message because you are subscribed to the Google Groups "mpir-devel" group. To unsubscribe from this group and stop receiving emails from it, send an email to mpir-devel+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/mpir-devel/CAB0xFnvNS%2B6Okk%3DiZ-DGCqQ0N4xf6LD-jvkLuSHGew9VvyznjA%40mail.gmail.com.
