Write array to text file

Question

I'm trying to write a 5x3 array to a text file following an example found here using this code.

import numpy as np
data = np.loadtxt('array_float.txt')
with open('out.txt', 'w') as outfile:
    for data_slice in data:
        np.savetxt(outfile, data_slice, fmt='%4.1f')

It results in the following error:

File C:\Python34\lib\site-packages\numpy\lib\npyio.py", line 1087, in savetxt
  fh.write(asbytes(format % tuple(row) + newline))
TypeError: must be str, not bytes

It seems savetxt doesn't like the outfile object. I'm able to get savetxt to work when I specify the actual outfile name. For example, this works:

np.savetxt('out.txt', data_slice, fmt='%4.1f')

But only the last line of the array gets save to 'out.txt'.

rafaelc · Accepted Answer · 2015-05-21 21:27:56Z

3

You should open the file in binary mode (using ab or wb)

import numpy as np
data = np.loadtxt('array_float.txt')
with open('out.txt', 'ab') as outfile:
    for data_slice in data:
        np.savetxt(outfile, data_slice, fmt='%4.1f')

answered May 21, 2015 at 21:27

rafaelc

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4 Comments

Dave Leighton Over a year ago

Thanks, that works but the array is all on one line in the output file. How do I get each line of the array to write to a new line in the output file?

rafaelc Over a year ago

@DaveLeighton Try np.savetxt(outfile, data_slice, fmt='%4.1f', newline="\n") instead

Dave Leighton Over a year ago

Yeah, that was the first thing I tried, but it didn't change the output. Still all on one line.

ShadowRanger Over a year ago

@RafaelCardoso: For portability to a Windows system with a file opened in binary mode, shouldn't that be something like newline=os.linesep? Windows text editors often treat \n by itself as plain whitespace or nothing, and only \r\n actually gets rendered as a line break. The default is already \n after all, so newline="\n" doesn't change behavior.

Douglas - 15 year old Pythoner · Accepted Answer · 2015-12-16 14:41:38Z

1

I suggest you use Python's pickle module. It allows the saving of any array with very few complications or lines of code.

Try this:

import pickle
f = open(name_of_file,'w')
pickle.dump(f,name_of_array)
f.close()

answered Dec 16, 2015 at 14:41

Douglas - 15 year old Pythoner

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