“如果没有”条件不按预期工作 [英] "If else" conditionals not working as expected
问题描述
#include<stdio.h>
int main(){
float x = 9.81 ;
if( x == 9.81)
{printf("the value of x is 9.81\n");}
else {printf("No its not 9.81 \n");}
}
在此，我们期待如果要执行的语句，而是else语句被执行。
为什么会这样？
In this we expect "if" statement to execute but instead "else" statement get executed.
Why is it so??
推荐答案
这是浮动的常见问题不是他们看起来的样子。 ;-)
原因是内存中有内部表示。输出它。
解决方案是计算差值，接受一个小的。请记住它可能已签名。
it is common issue that float arent exactly what they seem. ;-)
The reason is there internal representation in the memory. Make an output of it.
The solution is to calculate the difference an accept a small one. Remember that it maybe signed.
float x = 9.81f;
float d = x - 9.81f;
printf("the diff is %f\n", d);
if( fabs(d) < 0.005/*accepted tolerance*/ )
{printf("the value of x is 9.81\n");}
else {printf("No its not 9.81 \n");}
}
KarstenK 已经为您提供了解决方案。我建议你阅读：每个计算机科学家应该知道的关于浮点运算的内容 [ ^ ]。
KarstenK already gave you the solution. I suggest you to read: "What Every Computer Scientist Should Know About Floating-Point Arithmetic"[^].
除了其他答案，我只需要5美分：
使用运营商不是一个好主意==使用浮点操作数。这是对的，我不知道它可能有用的情况。即使浮点值表示零小数部分的数字，也不应该使用它;相反，你可以将值和类型转换为整数类型之一，甚至很少需要。浮点值表示近似值。如果不可能代表全部实数就像计算机这样的有限状态机，即使是单个实数，通常也包含无限量的信息。在有意义的情况下，典型的比较可能如下所示：
Just my 5 cents in addition to other answers:
It's not a good idea to use the operator == with floating-point operands. That's right, I don't know the situations where it can be useful at all. Even with floating-point values representing numbers with zero fractional part, you should not use it; instead you can round a value and typecast to one of the integer types, and even that is rarely needed. Floating-point values represent approximate values. If is impossible to represent "all" real number is such a finite-state machine as computer, even a single real number, generally, contains infinite volume of information. A typical comparison, in case when it makes sense, may look like:
#include <cmath>
double margin = //... some value which is "small enough"
double myValue = //...
double testValue = //...
// let's say, you mean to compare:
// if (myValue == testValue) ... // bad idea
// instead, do this:
if (std::abs(myValue - testValue) < margin) // ...
我提交这篇文章之后，我发现这个代码示例与解决方案1中的代码示例基本相同。 ，我决定不删除我的，也许它可以提供一些额外的澄清。
有时你需要一个不同的标准，相对接近。例如
After I submitted this post, I found that this code sample is essentially the same as in Solution 1. Crediting that, I decided not to remove mine, maybe it may provide some extra clarification.
Sometimes you need a different criterion, "relative closeness". For example
double margin = 0.001; // say, 0.1%
if (std::abs(myValue - testValue) * 2 <
margin * (myValue + testValue)) // ...
假设比较值是有用的是大值，它们之间的小差异，所以标准是检查相对差异是否足够小。
[END EDIT]
参见： std :: abs（ float），std :: fabs - cppreference.com [ ^ ]。
浮点计算的原理已在经典
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