問題描述

我正在讀取一個帶有浮點數(shù)的文本文件，所有這些都帶有 1 或 2 個小數(shù)點.我正在使用 float() 將一行轉(zhuǎn)換為浮點數(shù)，如果失敗則引發(fā) ValueError .我將所有浮點數(shù)存儲在一個列表中.打印出來時，我想將其打印為小數(shù)點后 2 位的浮點數(shù).

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.

假設(shè)我有一個帶有數(shù)字 -3,65, 9,17, 1 的文本文件.我閱讀了每個文件，并將它們轉(zhuǎn)換為浮點數(shù)并將它們附加到列表中.現(xiàn)在在 Python 2 中，調(diào)用 float(-3.65) 會返回 -3.65.然而，在 Python 3 中，float(-3.65) 返回-3.6499999999999999`，這會失去其精度.

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.

我想打印浮點數(shù)列表，[-3.6499999999999999, 9.1699999999999999, 1.0] 僅包含 2 個小數(shù)點.按照 '%.1f' % round(n, 1) 的行做一些事情會返回一個字符串.如何返回浮點數(shù)的所有兩個小數(shù)點的列表，而不是字符串?到目前為止，我使用 [round(num, 2) for num in list] 對其進行了四舍五入，但需要設(shè)置小數(shù)點/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().

推薦答案

一句話，你不能.

3.65 不能完全表示為 float.您得到的數(shù)字是與 3.65 最接近的數(shù)字，具有精確的 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.

(舊的?)Python 2 和 3 之間的區(qū)別純粹是由于默認格式.

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 中都看到了以下內(nèi)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'

有關(guān)精確的十進制數(shù)據(jù)類型，請參閱 decimal.Decimal.

For an exact decimal datatype, see decimal.Decimal.

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