Francois,
Never mind, I think I figured it out. Apparently, sage initializes .001 in
MPFR using the fact that .001 = 1/1000. Using the appropriate set methods
in MPFR fixes the issue. Attached is the code that makes Sage and MPFR
agree.
Apparently, if I treat .001 as a float or double in MPFR the value MPFR
returns does not agree with Sage. If I treat .001=1/1000 though, and
initialize the rational with mpq, then take the logarithm the output will
agree.
This makes sense I guess. Perhaps .001 is not exactly representable in
base 2 which causes the issue.
Thanks,
Rick
On Thursday, October 2, 2014 8:44:50 PM UTC-4, François wrote:
>
> Don't really know at this stage, I am not familiar enough with that area
> of sage to say whether or not your "a" will be defined on RR or a standard
> numpy real or something else.
>
> Francois
>
> On Thu, 02 Oct 2014 17:35:49 ref...@uncg.edu <javascript:> wrote:
> > Francois,
> >
> > Thank you for your reply. That's what I figured also, but where is the
> > difference between the two codes?
> >
> > Thanks,
> >
> > Rick
> >
> > On Thursday, October 2, 2014 8:32:45 PM UTC-4, François wrote:
> > > On Thu, 02 Oct 2014 17:28:15 ref...@uncg.edu <javascript:> wrote:
> > > > When comparing the digits though, it appears that only around the
> first
> > >
> > > 16
> > >
> > > > or so digits are the same.
> > >
> > > That's were you can infer that something has been converted or left
> > > as a double rather than a mpfr real number.
> > >
> > > Francois
>
>
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/*
* logs.c
*
* Author: Ricky Farr
*/
#include <stdio.h>
#include <stdlib.h>
#include <gmp.h>
#include <mpfr.h>
int main(){
mpfr_t x;
mpfr_init2(x,1000);
mpq_t a;
mpq_init(a);
mpq_set_ui(a,1,1000);
mpfr_set_q(x,a,MPFR_RNDN);
mpfr_log(x,x,MPFR_RNDN);
mpfr_out_str(stdout,10,0,x,MPFR_RNDN);
printf("\n");
//This code checks the value using the fact that e^log(.001) = .001
mpfr_exp(x,x,MPFR_RNDN);
mpfr_out_str(stdout,10,0,x,MPFR_RNDN);
printf("\n");
printf("\n");
return 0;
}
