Saving numpy array to txt file row wise

Question

I have an numpy array of form

a = [1,2,3]

which I want to save to a .txt file such that the file looks like:

1 2 3

If I use numpy.savetxt then I get a file like:

1
2
3

There should be a easy solution to this I suppose, any suggestions?

Or you only dealing with 1D arrays? – Benjamin Bannier Mar 05 '12 at 11:05 — Benjamin Bannier, Mar 05 '12 at 11:05

score 53 · Accepted Answer · edited Feb 08 '18 at 07:12

53

If numpy >= 1.5, you can do:

# note that the filename is enclosed with double quotes,
# example "filename.txt"

numpy.savetxt("filename", a, newline=" ")

Edit

several 1D arrays with same length

a = numpy.array([1,2,3])
b = numpy.array([4,5,6])
numpy.savetxt(filename, (a,b), fmt="%d")

# gives:
# 1 2 3
# 4 5 6

several 1D arrays with variable length

a = numpy.array([1,2,3])
b = numpy.array([4,5])

with open(filename,"w") as f:
    f.write("\n".join(" ".join(map(str, x)) for x in (a,b)))

# gives:
# 1 2 3
# 4 5

edited Feb 08 '18 at 07:12

kRazzy R

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answered Mar 05 '12 at 11:08

Avaris

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What if a new identical array is to be added to the file, at next row. How to break the line first line and continue on the second line? – Palle Mar 05 '12 at 16:46
1

@PatrikT: If you have more than one 1D arrays you can just do `numpy.savetxt(filename,(a,b,c))`. It saves row wise. But they should have same size. – Avaris Mar 05 '12 at 20:03
What if e.g. a is shorter than b and c? How do I save these 3 arrays row wise? – Palle Mar 07 '12 at 13:03
1

@PatrikT: If you have variable length arrays, `savetxt` is not much of help. It is possible to do but it gets uglier and beats the purpose I think. Just write them normally as [BioGeek](http://stackoverflow.com/a/9565547/843822) suggested in a loop. I'll edit my answer to include all those alternatives. – Avaris Mar 07 '12 at 20:25
`newline=" "` will result in a trailing delimiter (space), which some programs don't accept (there must be strictly a new line at the end of the printing, rather than a space and then a new line). – Serge Rogatch Oct 23 '22 at 05:34

score 26 · Answer 2 · answered Mar 05 '12 at 14:38

26

An alternative answer is to reshape the array so that it has dimensions (1, N) like so:

savetext(filename, a.reshape(1, a.shape[0]))

answered Mar 05 '12 at 14:38

Hooked

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This is exactly what you need if you're dumping readings into a file where every reading is made up of N samples. Perfect answer. – MedoAlmasry May 11 '20 at 00:24

score 9 · Answer 3 · answered Mar 05 '12 at 11:05

9

import numpy
a = numpy.array([1,2,3])

with open(r'test.txt', 'w') as f:
    f.write(" ".join(map(str, a)))

answered Mar 05 '12 at 11:05

BioGeek

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omegamanda · Answer 4 · 2016-05-22T19:54:27.973

6

I found that the first solution in the accepted answer to be problematic for cases where the newline character is still required. The easiest solution to the problem was doing this:

numpy.savetxt(filename, [a], delimiter='\t')

edited May 22 '16 at 19:54

answered May 13 '16 at 23:50

omegamanda

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score 3 · Answer 5 · edited Mar 15 '18 at 13:56

3

import numpy as np

a = [1,2,3]    
b = np.array(a).reshape((1,3))    
np.savetxt('a.txt',b,fmt='%d')

edited Mar 15 '18 at 13:56

Keyur Potdar

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answered Mar 15 '18 at 13:55

Innis

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While this code-only answer may solve the problem at hand, more explanation is necessary to help future users of the site understand how to apply this solution to their situation. – Keyur Potdar Mar 15 '18 at 13:57
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Please add some explanation to your answer – Ryan Schaefer Mar 15 '18 at 14:14

score 3 · Answer 6 · answered Nov 10 '18 at 07:17

3

I know this is old, but none of these answers solved the root problem of numpy not saving the array row-wise. I found that this one liner did the trick for me:

b = np.matrix(a)
np.savetxt("file", b)

answered Nov 10 '18 at 07:17

corym

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Would you explain "array row-wise"? – Cloud Cho Aug 30 '23 at 00:11

score 1 · Answer 7 · edited Sep 29 '18 at 17:06

1

Very very easy: [1,2,3]

A list is like a column.

1
2
3

If you want a list like a row, double corchete:

[[1, 2, 3]]  --->    1, 2, 3

and

[[1, 2, 3], [4, 5, 6]]  ---> 1, 2, 3
                             4, 5, 6

Finally:

np.savetxt("file", [['r1c1', 'r1c2'], ['r2c1', 'r2c2']], delimiter=';', fmt='%s')

Note, the comma between square brackets, inner list are elements of the outer list

edited Sep 29 '18 at 17:06

Matt C

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answered Sep 29 '18 at 16:37

McKinley

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score 0 · Answer 8 · answered Mar 06 '19 at 08:15

The numpy.savetxt() method has several parameters which are worth noting:

fmt : str or sequence of strs, optional
    it is used to format the numbers in the array, see the doc for details on formating

delimiter : str, optional
    String or character separating columns

newline : str, optional
    String or character separating lines.

Let's take an example. I have an array of size (M, N), which consists of integer numbers in the range (0, 255). To save the array row-wise and show it nicely, we can use the following code:

import numpy as np

np.savetxt("my_array.txt", my_array, fmt="%4d", delimiter=",", newline="\n")

score -1 · Answer 9 · answered Mar 05 '12 at 11:04

-1

just

' '.join(a)

and write this output to a file.

answered Mar 05 '12 at 11:04

neizod

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That will give a `TypeError: sequence item 0: expected string, numpy.int32 found`, so you must first convert to string before joining. – BioGeek Mar 05 '12 at 11:06
' '.join(str(x) for x in a) – st.ph.n Jun 07 '16 at 17:38

Saving numpy array to txt file row wise

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