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Asked: 2026-05-10T17:01:27+00:00

In the code below I use mpf_add to add the string representation of two

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In the code below I use mpf_add to add the string representation of two floating values. What I don’t understand at this point is why 2.2 + 3.2 = 5.39999999999999999999999999999999999999. I would have thought that gmp was smart enough to give 5.4.

What am I not comprehending about how gmp does floats?

(BTW, when I first wrote this I wasn’t sure how to insert a decimal point, thus the plus/minus digit stuff at the end)

BSTR __stdcall FBIGSUM(BSTR p1, BSTR p2 ) {   USES_CONVERSION;    F(n1);   F(n2);   F(res);    LPSTR sNum1 = W2A( p1 );   LPSTR sNum2 = W2A( p2 );    mpf_set_str( n1, sNum1, 10 );   mpf_set_str( n2, sNum2, 10 );    mpf_add( res, n1, n2 );    char * buff =  (char *) _alloca( 1024 );   char expBuffer[ 20 ];   mp_exp_t exp;    mpf_get_str(buff, &exp, 10, 0, res);    char * temp = ltoa( (long) exp, expBuffer, 10 );   if (exp >= 0) {     strcat(buff, '+' );   }   strcat(buff, expBuffer );    BSTR bResult = _com_util::ConvertStringToBSTR( buff );   return bResult; }
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1 Answer

  1. This is because of the inherent error of using floating-point arithmetic in a binary environment.

    See the IEEE 754 standard for more information.

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