How to write a numpy matrix in a text file - python

Question

Suppose I am getting a numpy matrix from some calculation. Here is my numpy matrix 'result1'::

    result1=
    [[   1.         0.         0.         0.00375   -0.01072   -0.      -1000.     ]
     [   2.         3.         4.         0.        -0.004    750.         0.     ]
     [   3.         3.         0.         0.         0.      -750.      1000.     ]]

Now I want to write this matrix in a text file named 'result.txt'. For this, I wrote the following code::

np.savetxt('result.txt', result1, fmt='%.2e')

But it is giving me all the elements of the matrix in one row.

    1.00e+00 0.00e+00 0.00e+00 3.75e-03 -1.07e-02 -1.14e-13 -1.00e+032.00e+00 3.00e+00 4.00e+00 0.00e+00 -4.00e-03 7.50e+02 0.00e+003.00e+00 3.00e+00 0.00e+00 0.00e+00 0.00e+00 -7.50e+02 1.00e+03

I want to write the matrix in the text file in the proper matrix format. How can I do this? I used keyword newline='\n' or newline='',but the result is same.

Thanks in advance...

=======

This edited part is for @Warren

try this one:

>>> import numpy as np
>>> mat=np.matrix([[1, 2, 3],[4, 5, 6],[7, 8, 9]])
>>> mat
matrix([[1, 2, 3],
        [4, 5, 6],
        [7, 8, 9]])
>>> np.savetxt('text.txt',mat,fmt='%.2f')

in my text.txt file, I am getting:

1.00 2.00 3.004.00 5.00 6.007.00 8.00 9.00

I can't reproduce this. If I give a 2-d array (or np.matrix) to savetxt, it creates a file with one line for each row of the array. Could you create a complete minimal example that we could run that gives the single line file? — Warren Weckesser
– Warren Weckesser, Commented Mar 1, 2014 at 19:45
[This comment is a reply to a comment that was deleted.] Your example works as expected for me (with both numpy 1.7.1 and 1.8.0). How are you viewing the file 'text.txt'? I'm suspicious about the lack of a space between 3.00 and 4.00 and between 6.00 and 7.00 in the output that you showed. You sample output in the question also has spaces missing where the line separators would be. (Also, what operating system are you running this on? Might not matter, but it wouldn't hurt to know.) — Warren Weckesser
– Warren Weckesser, Commented Mar 1, 2014 at 20:21
OK, thanks. My previous comment now applies to the change that you made to the question. — Warren Weckesser
– Warren Weckesser, Commented Mar 1, 2014 at 20:30
This looks related, although it only mentions python 2.7: github.com/numpy/numpy/pull/3976 (pull request) and github.com/numpy/numpy/issues/3975 (bug report). — Warren Weckesser
– Warren Weckesser, Commented Mar 1, 2014 at 20:58

DAYHU · Accepted Answer · 2017-08-03 16:43:15Z

8

like Francesco Nazzaro's answer, but a little different to make sure the file can be opened successfully, try:

a = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
mat = np.matrix(a)
with open('outfile.txt','wb') as f:
    for line in mat:
        np.savetxt(f, line, fmt='%.2f')

edited Aug 3, 2017 at 16:43

answered Aug 3, 2017 at 15:51

DAYHU

811 silver badge4 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Community · Accepted Answer · 2019-08-29 13:52:46Z

4

If you want to use only numpy:

import numpy as np

mat = np.matrix([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
with open('outfile.txt') as f:
    for line in mat:
        np.savetxt(f, line, fmt='%.2f')

and then

cat outfile.txt
1.00 2.00 3.00
4.00 5.00 6.00
7.00 8.00 9.00

Pandas has to_csv method:

import numpy as np
import pandas as pd

mat = np.matrix([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
df = pd.DataFrame(data=mat.astype(float))
df.to_csv('outfile.csv', sep=' ', header=False, float_format='%.2f', index=False)

it has the same output:

cat outfile.csv
1.00 2.00 3.00
4.00 5.00 6.00
7.00 8.00 9.00

edited Aug 29, 2019 at 13:52

CommunityBot

11 silver badge

answered Feb 5, 2016 at 9:18

Francesco Nazzaro

2,93614 silver badges21 bronze badges

Comments

dawg · Accepted Answer · 2014-03-01 20:28:13Z

To recreate the shape, you need to save the shape when you save the file.

Try:

import numpy as np
import re

result=np.array([[1.,0.,0.,0.00375,-0.01072,-0.,-1000.,],
                 [2.,3.,4.,0.,-0.004,750.,0.],
                 [3.,3.,0.,0.,0.,-750.,1000.]])

with open('/tmp/test', 'w') as fout:
    fout.write(u'#'+'\t'.join(str(e) for e in result.shape)+'\n')
    result.tofile(fout)

with open('/tmp/test', 'rb') as f:
    line=f.readline().decode('ascii')
    if line.startswith('#'):
        shape=tuple(map(int, re.findall(r'(\d+)', line)))
    else:
        raise IOError('Failed to find shape in file')    

    result2=np.fromfile(f)
    result3=result2.reshape(shape)

print(np.array_equal(result, result2))
# False
print(np.array_equal(result, result3))
# True

You can save the shape in some form in the file in oder to recreate the same shape. Make sure you do not forget the data at the beginning of the file however, since unlike np.loadtxt, lines starting with # are still considered data.

Vincenzo Fiorentini · Accepted Answer · 2025-01-23 17:22:18Z

0

this worked for me (python 3.12) to write lines of a matrix (a numpy array), with elements separated by spaces (you can use ','.join etc. to comma-separate).

file='nameoffile'
f=open(file,'w')
for line in arraytobesaved:
   line2=' '.join([str(x) for x in line])
#   line2=','.join([str(x) for x in line]) ### CSV
   f.write(line2+'\n')
f.close()

edited Jan 23 at 17:22

answered Jan 23 at 17:21

Vincenzo Fiorentini

11 bronze badge

Collectives™ on Stack Overflow

How to write a numpy matrix in a text file - python

4 Answers 4

Comments

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related