convert numpy array to string with given format

Question

I would like to convert a numpy array into a string representation with a given format. This

from io import BytesIO
import numpy

data = numpy.random.rand(5)

s = BytesIO()
numpy.savetxt(s, data, "%.15e")
out = s.getvalue().decode()

print(out)

3.208726298090422e-01
6.817590490300521e-01
3.446035342640975e-01
7.871066165361260e-01
4.829308426574872e-01

works, but savetxt is slow. tofile is about twice as fast, but I don't know how to get it to work with BytesIO. Perhaps there is another alternative.

Any hints?

savetxt creates a fmt string by replicating that %... to match the number of columns in data. It then does fmt % tuple(row). The Python string formatting mechanism works on individual numbers (and by extension tuples of numbers), not on arrays. — hpaulj
– hpaulj, Commented Aug 7, 2019 at 17:52
Just curious, instead of using BytesIO, wouldn't .npy files, i.e. np.save in byte encoded format do your job? — Sayandip Dutta
– Sayandip Dutta, Commented Aug 7, 2019 at 19:13

Victor Ruiz · Accepted Answer · 2019-08-07 20:58:16Z

2

You can do something like this:

import numpy as np

data = np.random.rand(5)
out ='\n'.join(map('{:.15e}'.format, data))
print(out)

Output example:

2.599889521964338e-02
8.936410392960248e-01
7.074905121425787e-01
4.318334519811902e-01
8.219700656108224e-01

answered Aug 7, 2019 at 20:58

Victor Ruiz

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