問題描述
我正在讀取一個帶有浮點數的文本文件，所有這些都帶有 1 或 2 個小數點.我正在使用 float() 將一行轉換為浮點數，如果失敗則引發 ValueError .我將所有浮點數存儲在一個列表中.打印出來時，我想將其打印為小數點后 2 位的浮點數.
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.
假設我有一個帶有數字 -3,65, 9,17, 1 的文本文件.我閱讀了每個文件，并將它們轉換為浮點數并將它們附加到列表中.現在在 Python 2 中，調用 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.
我想打印浮點數列表，[-3.6499999999999999, 9.1699999999999999, 1.0] 僅包含 2 個小數點.按照 '%.1f' % round(n, 1) 的行做一些事情會返回一個字符串.如何返回浮點數的所有兩個小數點的列表，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，但需要設置小數點/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.您得到的數字是與 3.65 最接近的數字，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'
有關精確的十進制數據類型，請參閱 decimal.Decimal.
For an exact decimal datatype, see decimal.Decimal.
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