在C中比较相同的浮点值 [英] Comparing Same Float Values In C

问题描述

可能重复：

浮点数与浮点数比较的奇怪输出

当我试图比较两个相同的 float 值时，它不会在下面的代码中输出相等的值：

  void main（）
 {
 float a = 0.7; 
 clrscr（）; 
 if（a <0.7）
 printf（value：％f，a）; 
 else if（a == 0.7）
 printf（equal values）; 
 else 
 printf（hello）; 
 getch（）; 
 
 
 
 $ b $ p 
 $ p 
 
 b 
 $ b 
  float a = 0.7; 
  
使用 double 常量 0.7 创建一个单精度数字（失去一些精度）：
 
 
  if a == 0.7）
  
会比较两个 double 精确数字（<$ 
 
 
转换double  0.7时丢失的精度将 a 提升为双精度时，code>到浮点 a 不会重新获得。
 
 
 
如果您将所有 0.7 值更改为 0.7f （强制浮动而不是双），或者如果你只是使一个一个双，它会正常工作 - 我很少使用浮动现在除非我有大量的数组，并且需要节省空间。
 
 
你可以看到这个动作：
  #include< stdio.h> 
 int main（void）{
 float f = 0.7; //双转换为浮动
 double d1 = 0.7; // double double double 
 double d2 = f; // float转换为double 
 
 printf（double：％.30f\\\
，d1）; 
 printf（double from float：％.30f\\\
，d2）; 
 
返回0; 
 
 
 
 
 
 $ b 
会输出类似的内容（稍作修改以显示差异）：
 双：0.6999999 | 99999999955591079014994 
。从浮子双：0.6999999 | 88079071044921875000000 
 \_超出这里不同。 
  
 

  
Possible Duplicate:

  strange output in comparison of float with float literal  
When I am trying to compare 2 same float values it doesn't print "equal values" in the following code : 
void main()
{
    float a = 0.7;
    clrscr();
    if (a < 0.7)
        printf("value :  %f",a);
    else if (a == 0.7)
        printf("equal values");
    else
        printf("hello");
    getch();
}
Thanks in advance.
解决方案
While many people will tell you to always compare floating point numbers with an epsilon (and it's usually a good idea, though it should be a percentage of the values being compared rather than a fixed value), that's not actually necessary here since you're using constants.

Your specific problem here is that:
float a = 0.7;
uses the double constant 0.7 to create a single precision number (losing some precision) while:
if (a == 0.7)
will compare two double precision numbers (a is promoted first).

The precision that was lost when turning the double 0.7 into the float a is not regained when promoting a back to a double.

If you change all those 0.7 values to 0.7f (to force float rather than double), or if you just make a a double, it will work fine - I rarely use float nowadays unless I have a massive array of them and need to save space.

You can see this in action with:
#include <stdio.h>
int main (void){
    float f = 0.7;    // double converted to float
    double d1 = 0.7;  // double kept as double
    double d2 = f;    // float converted back to double

    printf ("double:            %.30f\n", d1);
    printf ("double from float: %.30f\n", d2);

    return 0;
}
which will output something like (slightly modified to show difference):
double:            0.6999999|99999999955591079014994
double from float: 0.6999999|88079071044921875000000
                            \_ different beyond here.


                        
这篇关于在C中比较相同的浮点值的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！