CODEWITHSUNDEEP
pythonnumpyscipy
Fookatchu
Fookatchu

Reputation: 7465

How to set the precision on str(numpy.float64)?

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:

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()?

kind regards,

fookatchu

Upvotes: 19

Views: 44602

Answers (5)

Mike T
Mike T

Reputation: 43762

Numpy 1.14 and later have format_float_positional and format_float_scientific functions 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.

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.

These advanced formatters are based on the Dragon4 algorithm; see Ryan Juckett's Printing Floating-Point Numbers to read more on the subject.

Upvotes: 7

tillsten
tillsten

Reputation: 14888

You could use normal String formating, see: http://docs.python.org/library/string.html#formatspec

Example:

print '{:.2f}'.format(0.1234)  # '0.12'
print '{:.2e}'.format(0.1234)  # '1.23e-01'

Upvotes: 12

fabrica
fabrica

Reputation: 316

Also you can do:

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.

Upvotes: 0

David Heffernan
David Heffernan

Reputation: 613612

You can just use standard string formatting:

>>> x = 1.2345678
>>> '%.2f' % x
'1.23'

Upvotes: 18

Ned Batchelder
Ned Batchelder

Reputation: 376072

Instead of str(foo[0]), use "%.3f" % foo[0].

Upvotes: 3

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