用gmp添加浮游物给“正确的”结果，有点 [英] Adding floats with gmp gives "correct" results, sort of

问题描述

在下面的代码中，我使用 mpf_add 来添加两个浮点值的字符串表示。我现在不明白的是为什么 2.2 + 3.2 = 5.39999999999999999999999999999999999999 。我原以为 gmp 足够聪明，给 5.4 。

我不理解gmp是如何浮动的？

（顺便说一句，当我第一次写这个I不知道如何插入一个小数点，因此在最后加/减数字的东西）

$ pre $ BS $ __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）;
$ b $ char * temp = ltoa（（long）exp，expBuffer，10）;
if（exp> = 0）{
strcat（buff，+）;
}
strcat（buff，expBuffer）;

BSTR bResult = _com_util :: ConvertStringToBSTR（buff）;
返回bResult;

解决方案

在二进制环境中使用浮点运算的错误。

请参阅 IEEE 754 标准的更多信息。

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;
}

解决方案

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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