Floating Point Confusion

Rob Crowther weilawei at gmail.com
Wed Sep 19 18:44:44 CEST 2007

I'm trying to write a wrapper for GNU MP for the upcoming version of Python,
but I'm stuck at figuring out how to get the component parts of an mpf_t.

The current code I have works for most things, but values such as 0.01 don't
seem to exist at all!

Small example, which I've been using to try and figure things out:

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

#define MPF_DEFAULT_PREC 128
#define MPF_DEFAULT_SIGFIGS (size_t) 0

int main(int argc, char *argv[]) {
mpf_t x;
size_t sigfigs = MPF_DEFAULT_SIGFIGS;
unsigned long int base = MPF_DEFAULT_BASE;
unsigned long int whole = 0;
unsigned long int decimal = 0;
mp_exp_t exponent;
char *value = NULL;
char *s_whole = NULL;

mpf_set_d(x, (double) 0.01);

value = mpf_get_str(NULL, &exponent, base, sigfigs, x);

if (strncmp(value, (char *) "", 1) > 0) {
if (exponent > 0) {
s_whole = malloc(exponent);
strncpy(s_whole, value, exponent);
printf("%s", s_whole);

value += exponent;
if (strlen(value) > 0) printf(".%s\n", value);


return 0;

For the wrapper, I'd ideally like to return the base, the whole component,
and the decimal component. mpf_get_str seems to be the only way to do this,
but I'm having very little luck. mpf_get_d_2exp doesn't seem like it will
help particularly, since one of the reasons for wrapping GNU MP in the first
place was to avoid a loss of precision. If I multiplied things out to get
the components I want, there would be a loss of precision, as well as speed.


Thank you for your time in advance.

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