Python 如何在 str(numpy.float64) 上设置精度？
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How to set the precision on str(numpy.float64)?
提问by Fookatchu
i need to write a couple of numpy floats to a csv-file which has additional string content. therefore i dont use savetxt etc. with numpy.set_printoptions() i can only define the print behaviour, but not the str() behaviour. i know that i miss something and it cant be that hard, but i dont find a reasonable answer on the interwebs. maybe someone can point me in the right direction. heres some example code:
我需要将几个 numpy 浮点数写入具有附加字符串内容的 csv 文件。因此，我不将 savetxt 等与 numpy.set_printoptions() 一起使用，我只能定义打印行为，而不能定义 str() 行为。我知道我错过了一些东西并且不会那么难，但我在互联网上找不到合理的答案。也许有人可以指出我正确的方向。继承人一些示例代码：
In [1]: import numpy as np
In [2]: foo = np.array([1.22334])
In [3]: foo
Out[3]: array([ 1.22334])
In [4]: foo[0]
Out[4]: 1.2233400000000001
In [5]: str(foo[0])
Out[5]: '1.22334'
In [6]: np.set_printoptions(precision=3)
In [7]: foo
Out[7]: array([ 1.223])
In [8]: foo[0]
Out[8]: 1.2233400000000001
In [9]: str(foo[0])
Out[9]: '1.22334'
How do i convert np.float to a nicely formatted string, which i can feed to file.write()?
我如何将 np.float 转换为格式良好的字符串，我可以将其提供给 file.write()？
kind regards,
亲切的问候，
fookatchu
福卡丘
采纳答案by David Heffernan
You can just use standard string formatting:
您可以只使用标准字符串格式：
>>> x = 1.2345678
>>> '%.2f' % x
'1.23'
回答by tillsten
You could use normal String formating, see: http://docs.python.org/library/string.html#formatspec
您可以使用普通的字符串格式，请参阅：http: //docs.python.org/library/string.html#formatspec
Example:
例子：
print '{:.2f}'.format(0.1234) # '0.12'
print '{:.2e}'.format(0.1234) # '1.23e-01'
回答by Ned Batchelder
Instead of str(foo[0]), use "%.3f" % foo[0].
而不是str(foo[0])，使用"%.3f" % foo[0]。
回答by fabrica
Also you can do:
你也可以这样做：
precision = 2
str(np.round(foo[0], precision))
It had some advantages for me over the ('%.2f' % x) when I needed to do string a str(np.log(0.0)) which is neatly treated to "-inf" by numpy so you don't have to bother here.
当我需要做字符串 str(np.log(0.0)) 时，它对我来说比 ('%.2f' % x) 有一些优势，它被 numpy 巧妙地处理为“-inf”，所以你没有打扰这里。
回答by Mike T
Numpy 1.14 and later have format_float_positionaland format_float_scientificfunctions to format a floating-point scalar as a decimal string in positional or scientific notation, with control over rounding, trimming and padding. These functions offer much more control to the formatting than conventional Python string formatters.
numpy的1.14和以后有format_float_positional和format_float_scientific功能格式化浮点标如在位置或科学记数法十进制字符串，具有过四舍五入，修整和填充控制。与传统的 Python 字符串格式化程序相比，这些函数对格式化提供了更多的控制。
import numpy as np
x = np.float64('1.2345678')
print(np.format_float_positional(x)) # 1.2345678
print(np.format_float_positional(x, precision=3)) # 1.235
print(np.format_float_positional(np.float16(x))) # 1.234
print(np.format_float_positional(np.float16(x), unique=False, precision=8)) # 1.23437500
y = x / 1e8
print(np.format_float_scientific(y)) # 1.2345678e-08
print(np.format_float_scientific(y, precision=3, exp_digits=1)) # 1.235e-8
etc.
等等。
These advanced formatters are based on the Dragon4 algorithm; see Ryan Juckett's Printing Floating-Point Numbersto read more on the subject.
这些高级格式化程序基于 Dragon4 算法；请参阅 Ryan Juckett 的打印浮点数以阅读有关该主题的更多信息。
