Python 3浮点小数点/精度 [英] Python 3 Float Decimal Points/Precision

问题描述

我正在读取带有浮点数的文本文件，全部使用1或2个小数点。我使用 float（）将一行转换为一个浮点数，如果失败则引发一个 ValueError 。我将所有的花车存储在一个列表中。打印出来的时候，我想把它打印成小数点后2位。

9,17，1.我读了每一个，一旦我将它们转换为浮动，并将它们追加到列表中。现在在Python 2中，调用 float（-3.65）返回 -3.65 。然而，在Python 3中， float（-3.65）返回 -3.6499999999999999`，这就失去了它的精度。

打印浮点数列表， [ - 3.6499999999999999，9.1699999999999999，1.0] 只有2个小数点。沿着'％。1f'％round（n，1）的方向做一些事情会返回一个字符串。我如何返回浮点数的所有两个小数点的列表，而不是字符串？到目前为止，我已经使用 [round（num，2）for num in list] 对其进行了四舍五入，但是需要设置小数点/精度而不是 round（）。

解决方案

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3.65 不能完全表示为 float 。您得到的数字是距离 3.65 最近的数字，具有精确的 float 表示形式。

Python 2和3之间的差异纯粹是由于默认的格式化。

我看到以下在Python 2.7.3和3.3.0中：

  In [1]：3.65 
 Out [1]： 3.65 
 
 In [2]：'％.20f'％3.65 
 Out [2]：'3.64999999999999991118'
  

有关精确的十进制数据类型，请参阅 decimal.Decimal 。

I am reading a text file with floating point numbers, all with either 1 or 2 decimal points. I am using float() to convert a line into a float, and raising a ValueError if that fails. I am storing all floats in a list. When printing it out, I'd like to print it out as a 2 decimal places floating point.

Assume I have a text file with the numbers -3,65, 9,17, 1. I read each one, and once I convert them to float and append them to a list. Now in Python 2, calling float(-3.65) returns -3.65. In Python 3 however, float(-3.65) returns-3.6499999999999999` which loses its precision.

I want to print the list of floats, [-3.6499999999999999, 9.1699999999999999, 1.0] with 2 decimal points only. Doing something along the lines of '%.1f' % round(n, 1) would return a string. How can I return a list of all two decimal points of floats, and not strings? So far, I rounded it using [round(num, 2) for num in list] but would need to set the decimal points / precision instead of round().

解决方案

In a word, you can't.

3.65 cannot be represented exactly as a float. The number that you're getting is the nearest number to 3.65 that has an exact float representation.

The difference between (older?) Python 2 and 3 is purely due to the default formatting.

I am seeing the following both in Python 2.7.3 and 3.3.0:

In [1]: 3.65
Out[1]: 3.65

In [2]: '%.20f' % 3.65
Out[2]: '3.64999999999999991118'

For an exact decimal datatype, see decimal.Decimal.

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