C Primer Plus第121页说，浮点型只能用> 和 < 来比较，那么我要判断 1.5>1.4999怎么办？

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C Primer Plus第121页说，浮点型只能用> 和 < 来比较，那么我要判断 1.5>1.4999怎么办？

[Memory4ever]

浮点数精度不同，如果等的话很危险...

[赵4老师]

http://bbs.csdn.net/topics/390676437

[bear234]

5.01>=5.001 这当然可以啦 浮点数做比较运算的时候问题通常是这样产生的：1.23 == 1.23 这大多数时候是false 因为精度问题，你只给出小数后两位有效数字，而编译器处理double显然不止两位 在这个时候，正确的做法是： a - b <= 0.001 如果这个是true，则视为a==b 你可以通过修改0.001来确定一个可以满足你需要的精度

[苏叔叔]

直接 if(1.5 > 1.4999)

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引用 1 楼 zhangxiangDavaid 的回复:

直接 if(1.5 > 1.4999)

我说错了，我是说我要判断 5.01>=5.001怎么办？ 难道浮点型没法判断 >= <=了?

[苏叔叔]

照样使用 if(5.01 >= 5.001)

[gh_99]

浮点型应该能判断 >= <= 如果你有问题可以写代码直接运行检验一下。

[startstop2015]

引用 3 楼 zhangxiangDavaid 的回复:

照样使用 if(5.01 >= 5.001)

C PRIMER PLUS第5版，第121页说，浮点型只能用> 和 < 来比较

[paschen]

书想表达的意思是最好不要用==、！=这种运算符判断浮点数是否相等，因为可能有精度误差导致问题，判断相等建议判断两浮点数相减绝对值是否小于一个很小的数

[yajuie]

引用 6 楼 paschen的回复:

书想表达的意思是最好不要用==、！=这种运算符判断浮点数是否相等，因为可能有精度误差导致问题，判断相等建议判断两浮点数相减绝对值是否小于一个很小的数

学习了，刚入门学习，还真没注意这方面问题

[Saleayas]

1.5>1.4999 这个为true，不需要程序去判断。 如果你使用变量来判断，那么你的程序肯定会有精度的要求。 此时需要依据你的精度来判断。 在实际使用的过程中，同样一个结果，在不同的时候是不一致的。

[赵4老师]

仅供参考：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
float f;
double d;
void showf() {
    char bs[65];
    char b[65];
    unsigned char *p;
    char e[12];
    char *t;
    int ex;

    printf("\nf=%.6f\n",f);
    p=(unsigned char *)&f;
    printf("hex=%02X %02X %02X %02X\n",p[3],p[2],p[1],p[0]);
    ltoa(*(long *)&f,b,2);
    sprintf(bs,"%032s",b);
    printf("bin=%s\n",bs);
    printf("bin=%.1s %.8s   %s\n",bs,bs+1,bs+9);
    strncpy(e,bs+1,8);e[8]=0;
    ex=strtol(e,&t,2);
    printf("    %c %-4d-127 1.%s\n",(bs[0]=='0')?'+':'-',ex,bs+9);
    ex-=127;
    printf("    %c %-8d 1.%s\n",(bs[0]=='0')?'+':'-',ex,bs+9);
}
void showd() {
    char bs[65];
    char b[65];
    unsigned char *p;
    char e[12];
    char *t;
    int ex;

    printf("\nd=%.15lf\n",d);
    p=(unsigned char *)&d;
    printf("hex=%02X %02X %02X %02X %02X %02X %02X %02X\n",p[7],p[6],p[5],p[4],p[3],p[2],p[1],p[0]);
    _i64toa(*(__int64 *)&d,b,2);
    sprintf(bs,"%064s",b);
    printf("bin=%s\n",bs);
    printf("bin=%.1s %.11s   %s\n",bs,bs+1,bs+12);
    strncpy(e,bs+1,11);e[11]=0;
    ex=strtol(e,&t,2);
    printf("    %c %-6d-1023 1.%s\n",(bs[0]=='0')?'+':'-',ex,bs+12);
    ex-=1023;
    printf("    %c %-11d 1.%s\n",(bs[0]=='0')?'+':'-',ex,bs+12);
}
int main() {

    for (f=0.0f;f<=2.2f;f+=0.2f) showf();
    printf("------------\n");
    f=2.0f;showf();
    printf("-----------------------------------\n");
    for (d=0.0 ;d<=2.2 ;d+=0.2 ) showd();
    printf("------------\n");
    d=2.0 ;showd();

    return 0;
}
//
//f=0.000000
//hex=00 00 00 00
//bin=00000000000000000000000000000000
//bin=0 00000000   00000000000000000000000
//  + 0   -127 1.00000000000000000000000
//  + -127     1.00000000000000000000000
//
//f=0.200000
//hex=3E 4C CC CD
//bin=00111110010011001100110011001101
//bin=0 01111100   10011001100110011001101
//  + 124 -127 1.10011001100110011001101
//  + -3       1.10011001100110011001101
//
//f=0.400000
//hex=3E CC CC CD
//bin=00111110110011001100110011001101
//bin=0 01111101   10011001100110011001101
//  + 125 -127 1.10011001100110011001101
//  + -2       1.10011001100110011001101
//
//f=0.600000
//hex=3F 19 99 9A
//bin=00111111000110011001100110011010
//bin=0 01111110   00110011001100110011010
//  + 126 -127 1.00110011001100110011010
//  + -1       1.00110011001100110011010
//
//f=0.800000
//hex=3F 4C CC CD
//bin=00111111010011001100110011001101
//bin=0 01111110   10011001100110011001101
//  + 126 -127 1.10011001100110011001101
//  + -1       1.10011001100110011001101
//
//f=1.000000
//hex=3F 80 00 00
//bin=00111111100000000000000000000000
//bin=0 01111111   00000000000000000000000
//  + 127 -127 1.00000000000000000000000
//  + 0        1.00000000000000000000000
//
//f=1.200000
//hex=3F 99 99 9A
//bin=00111111100110011001100110011010
//bin=0 01111111   00110011001100110011010
//  + 127 -127 1.00110011001100110011010
//  + 0        1.00110011001100110011010
//
//f=1.400000
//hex=3F B3 33 34
//bin=00111111101100110011001100110100
//bin=0 01111111   01100110011001100110100
//  + 127 -127 1.01100110011001100110100
//  + 0        1.01100110011001100110100
//
//f=1.600000
//hex=3F CC CC CE
//bin=00111111110011001100110011001110
//bin=0 01111111   10011001100110011001110
//  + 127 -127 1.10011001100110011001110
//  + 0        1.10011001100110011001110
//
//f=1.800000
//hex=3F E6 66 68
//bin=00111111111001100110011001101000
//bin=0 01111111   11001100110011001101000
//  + 127 -127 1.11001100110011001101000
//  + 0        1.11001100110011001101000
//
//f=2.000000
//hex=40 00 00 01
//bin=01000000000000000000000000000001
//bin=0 10000000   00000000000000000000001
//  + 128 -127 1.00000000000000000000001
//  + 1        1.00000000000000000000001
//------------
//
//f=2.000000
//hex=40 00 00 00
//bin=01000000000000000000000000000000
//bin=0 10000000   00000000000000000000000
//  + 128 -127 1.00000000000000000000000
//  + 1        1.00000000000000000000000
//-----------------------------------
//
//d=0.000000000000000
//hex=00 00 00 00 00 00 00 00
//bin=0000000000000000000000000000000000000000000000000000000000000000
//bin=0 00000000000   0000000000000000000000000000000000000000000000000000
//  + 0     -1023 1.0000000000000000000000000000000000000000000000000000
//  + -1023       1.0000000000000000000000000000000000000000000000000000
//
//d=0.200000000000000
//hex=3F C9 99 99 99 99 99 9A
//bin=0011111111001001100110011001100110011001100110011001100110011010
//bin=0 01111111100   1001100110011001100110011001100110011001100110011010
//  + 1020  -1023 1.1001100110011001100110011001100110011001100110011010
//  + -3          1.1001100110011001100110011001100110011001100110011010
//
//d=0.400000000000000
//hex=3F D9 99 99 99 99 99 9A
//bin=0011111111011001100110011001100110011001100110011001100110011010
//bin=0 01111111101   1001100110011001100110011001100110011001100110011010
//  + 1021  -1023 1.1001100110011001100110011001100110011001100110011010
//  + -2          1.1001100110011001100110011001100110011001100110011010
//
//d=0.600000000000000
//hex=3F E3 33 33 33 33 33 34
//bin=0011111111100011001100110011001100110011001100110011001100110100
//bin=0 01111111110   0011001100110011001100110011001100110011001100110100
//  + 1022  -1023 1.0011001100110011001100110011001100110011001100110100
//  + -1          1.0011001100110011001100110011001100110011001100110100
//
//d=0.800000000000000
//hex=3F E9 99 99 99 99 99 9A
//bin=0011111111101001100110011001100110011001100110011001100110011010
//bin=0 01111111110   1001100110011001100110011001100110011001100110011010
//  + 1022  -1023 1.1001100110011001100110011001100110011001100110011010
//  + -1          1.1001100110011001100110011001100110011001100110011010
//
//d=1.000000000000000
//hex=3F F0 00 00 00 00 00 00
//bin=0011111111110000000000000000000000000000000000000000000000000000
//bin=0 01111111111   0000000000000000000000000000000000000000000000000000
//  + 1023  -1023 1.0000000000000000000000000000000000000000000000000000
//  + 0           1.0000000000000000000000000000000000000000000000000000
//
//d=1.200000000000000
//hex=3F F3 33 33 33 33 33 33
//bin=0011111111110011001100110011001100110011001100110011001100110011
//bin=0 01111111111   0011001100110011001100110011001100110011001100110011
//  + 1023  -1023 1.0011001100110011001100110011001100110011001100110011
//  + 0           1.0011001100110011001100110011001100110011001100110011
//
//d=1.400000000000000
//hex=3F F6 66 66 66 66 66 66
//bin=0011111111110110011001100110011001100110011001100110011001100110
//bin=0 01111111111   0110011001100110011001100110011001100110011001100110
//  + 1023  -1023 1.0110011001100110011001100110011001100110011001100110
//  + 0           1.0110011001100110011001100110011001100110011001100110
//
//d=1.600000000000000
//hex=3F F9 99 99 99 99 99 99
//bin=0011111111111001100110011001100110011001100110011001100110011001
//bin=0 01111111111   1001100110011001100110011001100110011001100110011001
//  + 1023  -1023 1.1001100110011001100110011001100110011001100110011001
//  + 0           1.1001100110011001100110011001100110011001100110011001
//
//d=1.800000000000000
//hex=3F FC CC CC CC CC CC CC
//bin=0011111111111100110011001100110011001100110011001100110011001100
//bin=0 01111111111   1100110011001100110011001100110011001100110011001100
//  + 1023  -1023 1.1100110011001100110011001100110011001100110011001100
//  + 0           1.1100110011001100110011001100110011001100110011001100
//
//d=2.000000000000000
//hex=3F FF FF FF FF FF FF FF
//bin=0011111111111111111111111111111111111111111111111111111111111111
//bin=0 01111111111   1111111111111111111111111111111111111111111111111111
//  + 1023  -1023 1.1111111111111111111111111111111111111111111111111111
//  + 0           1.1111111111111111111111111111111111111111111111111111
//
//d=2.200000000000000
//hex=40 01 99 99 99 99 99 99
//bin=0100000000000001100110011001100110011001100110011001100110011001
//bin=0 10000000000   0001100110011001100110011001100110011001100110011001
//  + 1024  -1023 1.0001100110011001100110011001100110011001100110011001
//  + 1           1.0001100110011001100110011001100110011001100110011001
//------------
//
//d=2.000000000000000
//hex=40 00 00 00 00 00 00 00
//bin=0100000000000000000000000000000000000000000000000000000000000000
//bin=0 10000000000   0000000000000000000000000000000000000000000000000000
//  + 1024  -1023 1.0000000000000000000000000000000000000000000000000000
//  + 1           1.0000000000000000000000000000000000000000000000000000
//