c - 为什么下面的代码中不是 a==0 呢？

Question

#include <stdio.h>
int main( )
{
    float a=1.0;
    long i;

    for(i=0; i<100; i++)
    {
        a = a - 0.01;
    }
    printf("%e\n",a);
}

结果是:6.59e-07

Answer 1

这是一个二进制 float ，而不是十进制数 - 因此您需要预料到舍入误差。请参阅本文中的基本部分:

What Every Programmer Should Know About Floating-Point Arithmetic

例如，值0.01在二进制浮点型中没有精确的表示。要在样本中获得“正确”结果，您必须四舍五入或使用十进制浮点类型(请参阅 Wikipedia):

Binary fixed-point types are most commonly used, because the rescaling operations can be implemented as fast bit shifts. Binary fixed-point numbers can represent fractional powers of two exactly, but, like binary floating-point numbers, cannot exactly represent fractional powers of ten. If exact fractional powers of ten are desired, then a decimal format should be used. For example, one-tenth (0.1) and one-hundredth (0.01) can be represented only approximately by binary fixed-point or binary floating-point representations, while they can be represented exactly in decimal fixed-point or decimal floating-point representations. These representations may be encoded in many ways, including BCD.