自己动手写C语言浮点数转换字符串函数

    前几天，应一个小友要求，写了几个字符串转换函数帮助其进行语言学习，自觉其中的几个函数还比较满意，故发布在此，可供初学者参考。

    浮点数转换字符串函数说简单也简单，说麻烦，也够烦心的，关键看你如何写了。简单点的几十行代码就行，复杂点的恐怕就的几百行代码了。如果还要考虑移植性、可读性甚至可维护性等就更麻烦的了。我一贯认为，一些事务性的项目应着重考虑移植性、可读性和可维护性等，而常用的带点系统性质的函数代码就应该以执行效率为主。

    本文的浮点数转换字符串函数还是比较复杂的，基本可算得上较低层次的转换。由于我已经习惯了用BCB写C/C++代码，因此我写的浮点数转换字符串函数是80位扩展精度浮点数的，但那个小友拿回去试了一下，说他用的VC不支持80位扩展精度浮点数，虽然能定义long double变量，但实际上还是64位的，我只好又重写了一个64位双精度浮点数的，2个版本使用条件编译，这也算得上是移植性吧，呵呵。

    下面是浮点数转换字符串函数的全部代码：    2个版本的代码加起来很长，但还有个自写的springf函数（下篇文章开始介绍）也要用到本文除FloatToSt函数外的全部代码。

//---------------------------------------------------------------------------#defineUSE_EXTENDED/**************************************************************************** 定义浮点数转换字符串结构                                                 ****************************************************************************/typedef struct{SHORTexponent;/* 指数（整数位数） */BYTEnegative;/* 负数（0正，1负）*/CHARdigits[21];/* 十进制整数字串 */}FloatRec;#defineF_DEFDECIMALS6#defineF_MAXDECIMALS100#ifdefUSE_EXTENDED#defineF_MAXPRECISION19#defineF_CONEXPONENT0x3ffftypedef long doubleEXTENDED, *PExtended, *PEXTENDED;#include <pshpack2.h>typedef struct{UINT64 mantissa;USHORT exponent;}_Extended;#include <poppack.h>static CONST _Extended _tab0[] ={{0x8000000000000000, 0x3FFF},/* 10**0 */{0xA000000000000000, 0x4002},/* 10**1 */{0xC800000000000000, 0x4005},/* 10**2 */{0xFA00000000000000, 0x4008},/* 10**3 */{0x9C40000000000000, 0x400C},/* 10**4 */{0xC350000000000000, 0x400F},/* 10**5 */{0xF424000000000000, 0x4012},/* 10**6 */{0x9896800000000000, 0x4016},/* 10**7 */{0xBEBC200000000000, 0x4019},/* 10**8 */{0xEE6B280000000000, 0x401C},/* 10**9 */{0x9502F90000000000, 0x4020},/* 10**10 */{0xBA43B74000000000, 0x4023},/* 10**11 */{0xE8D4A51000000000, 0x4026},/* 10**12 */{0x9184E72A00000000, 0x402A},/* 10**13 */{0xB5E620F480000000, 0x402D},/* 10**14 */{0xE35FA931A0000000, 0x4030},/* 10**15 */{0x8E1BC9BF04000000, 0x4034},/* 10**16 */{0xB1A2BC2EC5000000, 0x4037},/* 10**17 */{0xDE0B6B3A76400000, 0x403A},/* 10**18 */{0x8AC7230489E80000, 0x403E},/* 10**19 */{0xAD78EBC5AC620000, 0x4041},/* 10**20 */{0xD8D726B7177A8000, 0x4044},/* 10**21 */{0x878678326EAC9000, 0x4048},/* 10**22 */{0xA968163F0A57B400, 0x404B},/* 10**23 */{0xD3C21BCECCEDA100, 0x404E},/* 10**24 */{0x84595161401484A0, 0x4052},/* 10**25 */{0xA56FA5B99019A5C8, 0x4055},/* 10**26 */{0xCECB8F27F4200F3A, 0x4058},/* 10**27 */{0x813F3978F8940984, 0x405C},/* 10**28 */{0xA18F07D736B90BE5, 0x405F},/* 10**29 */{0xC9F2C9CD04674EDF, 0x4062},/* 10**30 */{0xFC6F7C4045812296, 0x4065}/* 10**31 */};static CONST _Extended _tab1[] ={{0x9DC5ADA82B70B59E, 0x4069},/* 10**32 */{0xC2781F49FFCFA6D5, 0x40D3},/* 10**64 */{0xEFB3AB16C59B14A3, 0x413D},/* 10**96 */{0x93BA47C980E98CE0, 0x41A8},/* 10**128 */{0xB616A12B7FE617AA, 0x4212},/* 10**160 */{0xE070F78D3927556B, 0x427C},/* 10**192 */{0x8A5296FFE33CC930, 0x42E7},/* 10**224 */{0xAA7EEBFB9DF9DE8E, 0x4351},/* 10**256 */{0xD226FC195C6A2F8C, 0x43BB},/* 10**288 */{0x81842F29F2CCE376, 0x4426},/* 10**320 */{0x9FA42700DB900AD2, 0x4490},/* 10**352 */{0xC4C5E310AEF8AA17, 0x44FA},/* 10**384 */{0xF28A9C07E9B09C59, 0x4564},/* 10**416 */{0x957A4AE1EBF7F3D4, 0x45CF},/* 10**448 */{0xB83ED8DC0795A262, 0x4639}/* 10**480 */};static CONST _Extended _tab2[] ={{0xE319A0AEA60E91C7, 0x46A3},/* 10**512 */{0xC976758681750C17, 0x4D48},/* 10**1024 */{0xB2B8353B3993A7E4, 0x53ED},/* 10**1536 */{0x9E8B3B5DC53D5DE5, 0x5A92},/* 10**2048 */{0x8CA554C020A1F0A6, 0x6137},/* 10**2560 */{0xF9895D25D88B5A8B, 0x67DB},/* 10**3072 */{0xDD5DC8A2BF27F3F8, 0x6E80},/* 10**3584 */{0xC46052028A20979B, 0x7525},/* 10**4096 */{0xAE3511626ED559F0, 0x7BCA}/* 10**4608 */};static CONST EXTENDED _conPrec = 1E19;#else// USE_EXTENDED#defineF_MAXPRECISION17#defineF_CONEXPONENT0x03fftypedef doubleEXTENDED, *PExtended, *PEXTENDED;static CONST UINT64 _tab0[] ={{0x3FF0000000000000},/* 10**0 */{0x4024000000000000},/* 10**1 */{0x4059000000000000},/* 10**2 */{0x408F400000000000},/* 10**3 */{0x40C3880000000000},/* 10**4 */{0x40F86A0000000000},/* 10**5 */{0x412E848000000000},/* 10**6 */{0x416312D000000000},/* 10**7 */{0x4197D78400000000},/* 10**8 */{0x41CDCD6500000000},/* 10**9 */{0x4202A05F20000000},/* 10**10 */{0x42374876E8000000},/* 10**11 */{0x426D1A94A2000000},/* 10**12 */{0x42A2309CE5400000},/* 10**13 */{0x42D6BCC41E900000},/* 10**14 */{0x430C6BF526340000},/* 10**15 */{0x4341C37937E08000},/* 10**16 */{0x4376345785D8A000},/* 10**17 */{0x43ABC16D674EC800},/* 10**18 */{0x43E158E460913D00},/* 10**19 */{0x4415AF1D78B58C40},/* 10**20 */{0x444B1AE4D6E2EF50},/* 10**21 */{0x4480F0CF064DD592},/* 10**22 */{0x44B52D02C7E14AF7},/* 10**23 */{0x44EA784379D99DB4},/* 10**24 */{0x45208B2A2C280291},/* 10**25 */{0x4554ADF4B7320335},/* 10**26 */{0x4589D971E4FE8402},/* 10**27 */{0x45C027E72F1F1281},/* 10**28 */{0x45F431E0FAE6D722},/* 10**29 */{0x46293E5939A08CEA},/* 10**30 */{0x465F8DEF8808B024}/* 10**31 */};static CONST UINT64 _tab1[] ={{0x4693B8B5B5056E17},/* 10**32 */{0x4D384F03E93FF9F5},/* 10**64 */{0x53DDF67562D8B363},/* 10**96 */{0x5A827748F9301D32},/* 10**128 */{0x6126C2D4256FFCC3},/* 10**160 */{0x67CC0E1EF1A724EB},/* 10**192 */{0x6E714A52DFFC679A},/* 10**224 */{0x75154FDD7F73BF3C},/* 10**256 */{0x7BBA44DF832B8D46},/* 10**288 */};static CONST EXTENDED _conPrec = 1E17;#endif// !USE_EXTENDEDstatic CONST UINT64 _cvttab[F_MAXPRECISION] ={#ifdefUSE_EXTENDED0xDE0B6B3A7640000, 0x16345785D8A0000, #endif0x02386F26FC10000, 0x0038D7EA4C68000, 0x0005AF3107A4000, 0x00009184E72A000,0x00000E8D4A51000, 0x00000174876E800, 0x0000002540BE400, 0x00000003B9ACA00,0x000000005F5E100, 0x000000000989680, 0x0000000000F4240, 0x0000000000186A0,0x000000000002710, 0x0000000000003E8, 0x000000000000064, 0x00000000000000A,0x000000000000001};#defineDECIMAL_EXP(exponent)((((exponent - F_CONEXPONENT) * 0x4d10) >> 16) + 1)static VOID AdjFloatDigits(UINT64 value, INT precision, INT decimals, FloatRec *rec){INT i;// value是F_MAXPRECISION位十进制整数，故从最高位开始转换为数字串for (i = 0; value; i ++){rec->digits[i] = (CHAR)((value / _cvttab[i]) | 0x30);value %= _cvttab[i];}memset(rec->digits + i, 0, F_MAXPRECISION - i);// 下面对数字串作精度处理// 如果总的精度小于0，数字串为空串，该数字转换为'0'if ((i = (rec->exponent + decimals)) < 0){rec->exponent = rec->negative = rec->digits[0] = 0;return;}if (i > precision) i = precision;// 如果精度位数小于18，同时该位大于等于'5'，四舍五入if (i < F_MAXPRECISION && rec->digits[i] >= '5'){do{rec->digits[i --] = 0;rec->digits[i] ++;}while (i >= 0 && rec->digits[i] > '9');if (i < 0){rec->digits[0] = '1';rec->exponent ++;}}// 否则，去掉数字串尾部多余的'0'else{if (i > F_MAXPRECISION) i = F_MAXPRECISION;do rec->digits[i --] = 0;while (i >= 0 && rec->digits[i] == '0');if (i < 0) rec->negative = 0;}}#ifdefUSE_EXTENDED// 解析扩展精度浮点数为十进制字符串，并存入浮点数记录中// 参数：浮点数指针，精度，小数位，浮点数记录VOID FloatResolve(PEXTENDED pvalue, INT precision, INT decimals, FloatRec *rec){INT power, exponent;EXTENDED val;// 79位：扩展精度浮点数符号位rec->negative = ((_Extended*)pvalue)->exponent >> 15;// 64-78位：扩展精度浮点数阶码(阶码 - 0x3fff = 二进制指数)rec->exponent = ((_Extended*)pvalue)->exponent & 0x7fff;// 阶码转换为十进制指数exponent = DECIMAL_EXP(rec->exponent);power = F_MAXPRECISION - exponent;if (!rec->exponent || power <= -5120)// *pvalue = 0{rec->negative = rec->digits[0] = 0;return;}if (rec->exponent == 0x7fff || power >= 5120)// *pvalue = nan or inf{if (!((*(LPBYTE)pvalue + 7) & 0x80) ||!(*(PUINT64)pvalue & 0x7fffffffffffffff) || power >= 5120){lstrcpyA(rec->digits, "INF");}else{rec->exponent ++;rec->negative = 0;lstrcpyA(rec->digits, "NAN");}return;}// 0-63位：扩展精度浮点数尾数转换成F_MAXPRECISION位十进制浮点整数格式val = *pvalue;*((LPBYTE)&val + 9) &= 0x7f;// val = fabs(*pvalue)if (power > 0)// if (power > 0) val *= (10**power){val *= *(PEXTENDED)&_tab0[power & 31];power >>= 5;// power /= 32;if (power){if (power & 15)val *= *(PEXTENDED)&_tab1[(power & 15) - 1];power >>= 4;// power /= 16;if (power)val *= *(PEXTENDED)&_tab2[power - 1];}}else if (power < 0)// if (power < 0) val /= (10**power){power = -power;val /= *(PEXTENDED)&_tab0[power & 31];power >>= 5;// power /= 32;if (power){if (power & 15)val /= *(PEXTENDED)&_tab1[(power & 15) - 1];power >>= 4;// power /= 16;if (power)val /= *(PEXTENDED)&_tab2[power - 1];}}val += 0.5;// 四舍五入if (val >= _conPrec){val /= 10;exponent ++;}rec->exponent = exponent;// 调整并转换扩展精度浮点数尾数的整数部分rec->digitsAdjFloatDigits(*(PUINT64)&val >> ((((_Extended*)&val)->exponent - 0x3fff) ^ 0x3f),precision, decimals, rec);}#else// USE_EXTENDED// 解析双精度浮点数为十进制字符串，并存入浮点数记录中// 参数：浮点数指针，精度，小数位，浮点数记录VOID FloatResolve(PEXTENDED pvalue, INT precision, INT decimals, FloatRec *rec){INT power, exponent;EXTENDED val;// 63位：双精度浮点数符号位rec->negative = *((LPBYTE)pvalue + 7) >> 7;// 52-62位：双精度浮点数阶码(阶码 - 0x3ff = 二进制指数)rec->exponent = (*(PUINT64)pvalue >> 52) & 0x7ff;// 阶码转换为十进制指数exponent = DECIMAL_EXP(rec->exponent);power = F_MAXPRECISION - exponent;if (!rec->exponent || power <= -320)// *pvalue = 0{rec->negative = rec->digits[0] = 0;return;}if (rec->exponent == 0x7ff || power >= 320)// *pvalue = nan or inf{if ((*(PUINT64)pvalue & 0xfffffffffffff) == 0 || power >= 320){lstrcpyA(rec->digits, "INF");}else{rec->exponent ++;rec->negative = 0;lstrcpyA(rec->digits, "NAN");}return;}// 0-51位：双精度浮点数尾数转换成F_MAXPRECISION位十进制浮点整数格式val = *pvalue;*((LPBYTE)&val + 7) &= 0x7f;// val = fabs(*pvalue)if (power > 0)// if (power > 0) val *= (10**power){val *= *(PEXTENDED)&_tab0[power & 31];power >>= 5;// power /= 32;if (power)val *= *(PEXTENDED)&_tab1[power - 1];}else if (power < 0)// if (power < 0) val /= (10**power){power = -power;val /= *(PEXTENDED)&_tab0[power & 31];power >>= 5;// power /= 32;if (power)val /= *(PEXTENDED)&_tab1[power - 1];}// 16位十进制浮点整数四舍五入val += 0.5;if (val >= _conPrec){val /= 10;exponent ++;}rec->exponent = exponent;// 调整并转换扩展精度浮点数尾数的整数部分rec->digits// 清除52-63位，加隐藏的高位，F_MAXPRECISION=17，高位超过52位，所以左移AdjFloatDigits(((*(PUINT64)&val & 0x000fffffffffffff) | 0x0010000000000000) <<-(52 - ((*(PUINT64)&val >> 52) - 0x3ff)), precision, decimals, rec);}#endif// !USE_EXTENDED// 输出指数字符串到buffer，返回指数字符串长度INT PutExponent(LPSTR buffer, CONST FloatRec *rec){LPSTR p = buffer;INT e, exp = rec->digits[0]? rec->exponent - 1 : 0;*p ++ = rec->negative & 0x80? 'E' : 'e';if (exp < 0){exp = -exp;*p ++ = '-';}else *p ++ = '+';if ((e = (exp / 1000)) != 0){*p ++ = e + 0x30;exp %= 1000;}*p ++ = exp / 100 + 0x30;exp %= 100;*(PUSHORT)p = (((exp % 10) << 8) | (exp / 10)) + 0x3030;return (INT)(p - buffer + 2);}// 浮点数转换为字符串。参数：字符串，浮点数LPSTR FloatToStr(LPSTR str, EXTENDED value){INT exp;FloatRec rec;LPSTR pd = rec.digits;LPSTR ps = str;// 解析浮点数，并将信息保存在recFloatResolve(&value, 15, 9999, &rec);// 打印负数符号if (rec.negative) *ps ++ = '-';// NAN or INFif (*pd > '9'){memcpy(ps, pd, 4);return str;}exp = rec.exponent;// 如果十进制指数大于15或者小于-3，转换为指数形式if (exp > 15 || exp < -3){*ps ++ = *pd ++;if (*pd)for (*ps ++ = '.'; *pd; *ps ++ = *pd ++);ps += PutExponent(ps, &rec);*ps = 0;return str;}// 否则，转换为小数形式if (exp <= 0){*ps ++ = '0';if (*pd){for (*ps ++ = '.'; exp < 0; *ps ++ = '0', exp ++);while (*ps ++ = *pd ++);}else *ps = 0;}else{for (; exp > 0 && *pd; *ps ++ = *pd ++, exp --);if (*pd){*ps ++ = '.';while (*ps ++ = *pd ++);}else{memset(ps, '0', exp);ps[exp] = 0;}}return str;}

    代码开头的USE_EXTENDED为编译条件，如果你的编译系统不支持80位扩展精度浮点数，可将该定义注释掉。

    前面说了，由于该代码主要是学习用的，因此数据转换层次较低，涉及到的有关浮点数格式的知识，可在网上搜索到。当然，知道是一回事，而具体怎样去操作则又是另一回事了，一些关键地方，我都作了较详细的注释，相信能对初学者正确理解浮点数格式有所帮助。例如，如何在不调用C有关函数，不使用汇编而快速的求一个浮点数的绝对值？本文代码就直接对浮点数进行操作：*((LPBYTE)&val + 7) &= 0x7f;（双精度浮点数）和*((LPBYTE)&val + 9) &= 0x7f;（扩展精度浮点数），也就是直接将浮点数的最高位置零，这当然比什么if (val < 0) val = -val语句快多了。不过，如果你要说后者的可移植性好，那我就无话可说了。要说可移植性，前者也可办到的，修改一下：*((LPBYTE)&val + sizeof(val) - 1) &= 0x7f;不就行了么，除非浮点数格式规则改变。只不过本文代码主要用来学习，用显式的方式更有意义。

    80位扩展精度浮点数的有效数字为19位，而64位双精度浮点数有效数字为15 - 16位，本文的解析函数FloatResolve分别用了19位和17位的最大转换精度，尽可能的多显示几位，这主要是为自写的sprintf函数（另文介绍）做准备的，本文的浮点数转换字符串函数FloatToStr只用了最大15位的精度。

    下面是个很简单的调用例子：

int main(int argc, char* argv[]){CHAR s[32];UINT64 inf = 0x7ff0000000000000;double v = -1.230E45;INT a, b;puts(FloatToStr(s, -10000));puts(FloatToStr(s, 123456789012345678));puts(FloatToStr(s, 1234567890.12345678));puts(FloatToStr(s, 0.0001234567890));puts(FloatToStr(s, -0.00001234567890));puts(FloatToStr(s, v));puts(FloatToStr(s, 0));puts(FloatToStr(s, *(double*)&inf));system("pause");return 0;}

    下面是运行结果：

-100001.23456789012346e+0171234567890.123460.000123456789-1.23456789e-005-1.23e+0450INF请按任意键继续. . .

    再次声明：本文代码主要供学习使用，如作其它用途，出问题慨不负责。

    水平有限，错误在所难免，欢迎指正和指导。邮箱地址：[e-mail]