Python 3 浮点小数点/精度

问题描述

我正在读取一个带有浮点数的文本文件，所有这些都带有 1 或 2 个小数点.我正在使用 float() 将一行转换为浮点数，如果失败则引发 ValueError .我将所有浮点数存储在一个列表中.打印出来时，我想将其打印为小数点后 2 位的浮点数.

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.

假设我有一个带有数字 -3,65, 9,17, 1 的文本文件.我阅读了每个文件，并将它们转换为浮点数并将它们附加到列表中.现在在 Python 2 中，调用 float(-3.65) 会返回 -3.65.然而，在 Python 3 中，float(-3.65) 返回-3.6499999999999999`，这会失去其精度.

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.

我想打印浮点数列表，[-3.6499999999999999, 9.1699999999999999, 1.0] 仅包含 2 个小数点.按照 '%.1f' % round(n, 1) 的行做一些事情会返回一个字符串.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，我使用 [round(num, 2) for num in list] 对其进行了四舍五入，但需要设置小数点/精度而不是 round().

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().

推荐答案

一句话，你不能.

3.65 不能完全表示为 float.您得到的数字是与 3.65 最接近的数字，具有精确的 float 表示.

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.

(旧的?)Python 2 和 3 之间的区别纯粹是由于默认格式.

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

我在 Python 2.7.3 和 3.3.0 中都看到了以下内容:

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'

有关精确的十进制数据类型，请参阅 decimal.Decimal.

For an exact decimal datatype, see decimal.Decimal.

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