emmmmm, i just hope mpir tries to add 0s at the end when conversion. i tried mpf_set_str(), it seems give me correct result.
wei. 在 2020年8月10日星期一 UTC+8下午4:24:43，Bill Hart写道： > > Correction: integers and powers of 10 with non-negative exponent are > exactly representable in binary. > > On Mon, 10 Aug 2020 at 10:22, Bill Hart <goodwi...@googlemail.com > <javascript:>> wrote: > > > > On Mon, 10 Aug 2020 at 10:08, wei zhao <zwleop...@gmail.com > <javascript:>> 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-...@googlegroups.com <javascript:>. > > > 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/095760f7-9477-40e7-9e96-015df643e7c4o%40googlegroups.com.
