mpq_get_d() rounding for different platforms?

[K Shen]

Hi,

I ran the same code on all systems:

main () {
  double i, j, k;
  mpq_t rat;
 
  printf("hi\n");
  mpq_init(rat);
  mpq_set_ui(rat, 4, 5);
  k = mpq_get_d(rat);
  printf("rat is %.17f\n", k);
 
}

and I built from the same GMP source. 

The version of gcc used for each build is different: 

32 bit Linux                     gcc 3.2.3
MinGW 32 bit Windows    gcc 3.4.1
MinGW 64 bit Windows    gcc 4.4.0

Does the version of gcc makes a difference? The Linux version was the only one I was able to run make check on (and it passed, as far as I can tell), because the others were all cross-compiled. 

Is this a known issue for GMP 4.2.2 on Linux? Is there anyway I can avoid it -- I would like to use the same version of GMP for all platforms if possible.

Cheers,

Kish

--- On Tue, 3/17/09, Sisyphus <sisyphus1 at optusnet.com.au> wrote:
From: Sisyphus <sisyphus1 at optusnet.com.au>
Subject: Re: mpq_get_d() rounding for different platforms?
To: "K Shen" <kishshen at yahoo.com>, gmp-discuss at gmplib.org
Date: Tuesday, March 17, 2009, 10:56 AM

----- Original Message ----- From: "K Shen"
<kishshen at yahoo.com>

> For example, for the rational 4/5,
> 
> the MinGW version returns 0.79999999999999993, while the Linux version
> returns 0.80000000000000004.

Make sure that the assignment of 4/5 is being carried out in the same way.

As the following little program demonstrates, mpq_set_str(q, "4/5",
10) and mpq_set_d(q, (double) 4 / 5) assigm different values:

#############################
#include <stdio.h>
#include <gmp.h>

int main(void) {
   mpq_t q1;
   mpq_t q2;

   mpq_init(q1);
   mpq_init(q2);

   mpq_set_str(q1, "4/5", 10);
   mpq_set_d(q2, (double)4 / 5);

   printf("%.17f\n", mpq_get_d(q1));
   printf("%.17f\n", mpq_get_d(q2));

   mpq_clear(q1);
   mpq_clear(q2);
   return 0;
}
#############################

For me (both Linux and MinGW) that outputs:

0.79999999999999993
0.80000000000000004

Cheers,
Rob 



      
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://gmplib.org/list-archives/gmp-discuss/attachments/20090317/a4f1e211/attachment.html>