Understand numpy.savetxt() for Beginner with Examples – NumPy Tutorial

By | October 16, 2019

numpy.savetxt() function can help us to save numpy data into a txt file (.txt or .csv). In this tutorial, we will write some examples to help numpy beginners to undstand and use it correctly.

Syntax

numpy.savetxt(fname, X, fmt='%.18e', delimiter=' ', newline='n', header='', footer='', comments='# ', encoding=None)

Save an array to a text file.

Parameters

fname: the name of text file.

X: numpy 1D or 2D ndarray, this is very important, 3D, 4D can not be saved.

fmt: format the data in X, for example: %d or %10.5f

delimiter: string or character separating columns in fname.

newline: string or character separating lines.

header: string that will be written at the beginning of the file. It is often be used to a comments.

footer: string that will be written at the end of the file.

comments: string that will be prepended to the header and footer strings, you can use #, ;

encoding: the string encoding in fname, it can be utf-8.

Here we will write some examples to show how to use this function.

Create a 1-D numpy array

import numpy as np

data = np.array([1, 2, 3, 4, 5])
print(data)

Save 1-D numpy array into txt file

fname= 'data.txt'
np.savetxt(fname, data, fmt = '%d')

Open data.txt, we will find the content in data.txt is:

1
2
3
4
5

From the result, we can find, each element in 1-D array data will be a line in data.txt. For example, 1 in data variable and it will hold a line in data.txt.

If you want to save all elements in data variable into data.txt with  float type, you can do like this.

fname= 'data.txt'
np.savetxt(fname, data, fmt = '%f')

Then the content in data.txt is:

1.000000
2.000000
3.000000
4.000000
5.000000

Add header and footer to save

We can add some comments in our data.txt file to help others to understand our data. We can do like this.

fname= 'data.txt'
header = "this is header tip"
footer = 'this is foot tip'
np.savetxt(fname, data, fmt = '%f',header= header, footer= footer)

The content in data.txt is:

# this is header tip
1.000000
2.000000
3.000000
4.000000
5.000000
# this is foot tip

Save 2-D numpy array to txt file

Here we will save 2-D numpy array into a file, you should notice data in each line of data.txt, the content of each line is 1-D array in 2-D array.

import numpy as np

data = np.array([[1, 2, 3], [4, 5, 6]])
print(data)

fname= 'data.txt'
np.savetxt(fname, data, fmt = '%d')

The content in data.txt is:

1 2 3
4 5 6

Save 3-D array to txt file

numpy.savetxt() function only allow to save 1-D or 2-D array, if you save 3-D, you will get error. Here is an example.

import numpy as np

data = np.array([[[1], [2], [3]], [[4], [5], [6]]])
print(data)

fname= 'data.txt'
np.savetxt(fname, data, fmt = '%d')

Then you will get error: ValueError: Expected 1D or 2D array, got 3D array instead

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