在c ++中setprecision是否圆？如果是这样为什么我看到这个？ [英] Does setprecision in c++ round? If so why am I seeing this?

问题描述

以下片段在我预期为0.29848时输出0.29847：

The following snippet outputs 0.29847 when I would have expected 0.29848:

double f = 0.298475;
cout << setprecision(5) << f << endl;

然而，对于其他示例，我观察四舍五入：

For other examples, however, I observe rounding:

double f = 0.123459;
cout << setprecision(5) << f << endl;

输出：0.12346

outputs: 0.12346

p>

and

double f = 0.123454;
cout << setprecision(5) << f << endl;

输出：0.12345

outputs: 0.12345

推荐答案

0.298475 的数字不能在 double 其分母是2的幂，为11939/40000），并且存储的实际号实际上更接近0.29847而不是0.29848。

The number 0.298475 isn't representable exactly in a double (since it's not a fraction whose denominator is a power of two, being 11939/40000), and the actual number that is stored is actually closer to 0.29847 than to 0.29848.

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