I want to set the BigFloat(7.0687866210937500e-1) by using a precision of 15 bits and two different rounding modes (up and down). So I do: setprecision(15) setrounding(BigFloat, RoundDown) # or RoundUp BigFloat(7.0687866210937500e-1) and I get both times the same result: 0.706879. However, usin…

The mpfr sprintf-functions takes rounding mode in their specifications, so it should be possible to use them. GNU MPFR 4.2.1 How to install and use GNU MPFR, a library for reliable multiple precision floating-point arithmetic, version 4.2.1.

Often when working with BigFloat literals you want to be using strings — floating point literals are immediately Float32 or Float64 before going to the BigFloat constructor. But that’s not the problem here, because you’re using fewer bits. julia> setprecision(15) 15 julia> BigFloat("7.06878662109…

Thanks for your reply ! So the rounding down or up concerns only the passage from string to a Float32 or Float64 ? How can I get in julia the analog of MPFR_RNDD and MPFR_RNDU (that I use in C), when creating the BigFloat ?

[image] chkat: 7.0687866210937500e-1 This is exactly representable with a precision of 15 bits, so the rounding mode doesn’t matter (because there’s no rounding). I think this is just some kind of printing issue. julia> setprecision(10000) 10000 julia> big = BigFloat("7.0687866210937500e-1"…

[image] chkat: So I do: BTW please use code block Markdown to make your post readable. Finds instructions and other tips here: [image] Please read: make it easier to help you Meta Discussion Welcome to the Julia Discourse! We are enthusiastic about helping Julia pr…

[image] nsajko: I think this is just some kind of printing issue. Yeah. Almost all functions in the MPFR API take rounding modes, including the output functions. So I guess everything here is as expected, although the initial post was unclear. What exactly is your goal, @chkat ?

It confuses me how it happens that the printing in C, using two different rounding modes, gives two different decimals (0.706878 and 0.706879) but in Julia there is no difference: julia> BigFloat(7.0687866210937500e-1, RoundDown, precision=15) 0.706879 julia> BigFloat(7.0687866210937500e-1, RoundU…

Can you post your C-program? As noted above, the number 7.0687866210937500e-1 == 23163/2^15, so ideally no rounding should take place as demonstrated by @nsajko above. Unless “precision” means different things in the C-program and in julia.

[image] chkat: the printing in C, using two different rounding modes, gives two different decimals (0.706878 and 0.706879) This is OK, and the Julia code is also OK. I think you’re just missing (sorry for not being clear before) the distinction between printing and construction in Julia. In y…

#include<mpfr.h> int main(){ mpfr_t a; mpfr_init2(a, 15); mpfr_set_d(a, 7.0687866210937500e-1, MPFR_RNDD); mpfr_out_str(stdout,10, 0, a, MPFR_RNDD); // gives 7.06878e-1 mpfr_out_str(stdout,10, 0, a, MPFR_RNDU); // gives 7.06879e-1 }

Ah, ok. So in the C-program the float isn’t rounded, but you specify that the output should be 10 characters wide. Consequently, some decimals may have to be suppressed. The rounding specifies what should be done with the preceding decimal. Should it be rounded up or down? In julia, you specify how…

Issue with BigFloat rounding

New to Julia

nsajko July 9, 2024, 4:38pm 5

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Issue with BigFloat rounding

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