Question

I’m summing two negative floats:

char * lhs = '-2234.6016114467412141'; char * rhs = '-4939600281397002.2812'; 

According to Perl, using bignum and Math::BigFloat, the answer is

-4939600281399236.8828114467412141 

However, according to GMP, using the code below, the answer is

-4939600281399236.88281  

Where have I gone wrong? What happened to the remaining ‘14467412141’?

#include 'stdafx.h' #include 'gmp-static\gmp.h' #include <stdlib.h>         /* For _MAX_PATH definition */ #include <stdio.h> #include <malloc.h> #include <math.h>  #define F(x) mpf_t x; mpf_init( x );  void main(void) {     F(f_lhs);     F(f_rhs);     F(f_res);      char * resbuff;      mp_exp_t exp;      char * lhs = '-2234.6016114467412141';     char * rhs = '-4939600281397002.2812';      int validOp = mpf_set_str( f_lhs, lhs, 10 );     validOp = mpf_set_str( f_rhs, rhs, 10 );      mpf_add( f_res, f_lhs, f_rhs );      resbuff = mpf_get_str( NULL, &exp, 10, 0, f_res );     printf( 'Using mpf_add, %s + %s = %s (exp=%d)\n', lhs, rhs, resbuff, exp );      free(resbuff); } 

Sample output:

Using mpf_add, -2234.6016114467412141 + -4939600281397002.2812 = -493960028139923688281 (exp=16) 

P.S. I have tried adding a call to mpf_set_default_prec with larger than 64 (the default) values, but to no effect.

Answer 1

Looks like you are overflowing the mantissa at 64 bits. Try doing mpf_get_prec(f_res) to check it is the precision you want. If not call the mpf_set_default_prec() before you initialize any mpf vars (line 1 of main).