意外的结果等于浮点数

问题描述

问题不是为什么 0.1 + 0.9 不等于 1.0 。它有关不同的行为等于。

有人可以解释为什么下面的例子工作不同。

  float q = 0.1f; 
浮点数w = 0.9f; 
 float summ = q + w; 
 
 q + w =​​= 1.0f; //假
 summ == 1.0f; // True 
  

为什么operator == 这个问题是由于中间计算是以更高的精度执行的，而这个规则是为了解决这个问题。什么时候回到 float precision在每种情况下都是不同的。 根据

float summ = q + w 是一个赋值，因此被舍入到最近的 floa t ，在这种情况下，它是1.

q + w =​​= 1.0f 既不是一个强制转换，赋值或函数调用，所以加法的结果仍然是一个扩展的精度浮点数，它接近但不等于1。

Question is not about why 0.1 + 0.9 is not equals 1.0. Its about different behaviour of a equals.

Can someone explain why examples below works differently.

float q = 0.1f;
float w = 0.9f;
float summ = q + w;

q + w == 1.0f; // False
summ == 1.0f; // True

Why operator == works differently?

解决方案

The problem is due to the fact that the intermediate calculations are being performed in a higher precision, and the rules for when to round back to float precision are different in each case.

According to the docs

float summ = q + w is an assignment, and hence is rounded to the nearest float, which in this case is 1.

q + w == 1.0f is neither a cast, assignment or function call, so the result of the addition is still an extended precision float, which is close, but not equal, to 1.

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