限制浮点数到两个小数点 [英] Limiting floats to two decimal points

问题描述

我想将 a 四舍五入为13.95 。

I want a to be rounded to 13.95.

>>> a
13.949999999999999
>>> round(a, 2)
13.949999999999999

round 函数不能按照我的预期工作。 / p>

The round function does not work the way I expected.

推荐答案

您遇到了所有数字都无法表示的浮点数旧问题。命令行仅显示内存中的完整浮点格式。在浮点上，你的四舍五入的版本是相同的数字。由于计算机是二进制的，它们将浮点数存储为整数，然后将其除以2的幂，因此13.95将以类似于125650429603636838 /（2 ** 53）的方式表示。双精度数的精度为53位（16位），常规浮点数的精度为24位（8位）。 python中的浮点数使用双精度来存储值。

You are running into the old problem with floating point numbers that all numbers cannot be represented. The command line is just showing you the full floating point form from memory. In floating point your rounded version is the same number. Since computers are binary they store floating point numbers as an integer and then divide it by a power of two so 13.95 will be represented in a similar fashion to 125650429603636838/(2**53). Double precision numbers have 53 bits (16 digits) of precision and regular floats have 24 bits (8 digits) of precision. The floating point in python uses double precision to store the values.

例如

for example

  >>> 125650429603636838/(2**53)
  13.949999999999999

  >>> 234042163/(2**24)
  13.949999988079071

  >>> a=13.946
  >>> print(a)
  13.946
  >>> print("%.2f" % a)
  13.95
  >>> round(a,2)
  13.949999999999999
  >>> print("%.2f" % round(a,2))
  13.95
  >>> print("{0:.2f}".format(a))
  13.95
  >>> print("{0:.2f}".format(round(a,2)))
  13.95
  >>> print("{0:.15f}".format(round(a,2)))
  13.949999999999999

如果您的货币只有两位小数，那么您有两个更好的选择，使用整数和商店价值以美分为单位，然后除以100转换为美元。或者使用像十进制的定点数字

If you are after only two decimal places as in currency then you have a couple of better choices use integers and store values in cents not dollars and then divide by 100 to convert to dollars. Or use a fixed point number like decimal

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