How to use python numpy.savetxt to write strings and float number to an ASCII file?

Question

I have a set of lists that contain both strings and float numbers, such as:

import numpy as num

NAMES  = num.array(['NAME_1', 'NAME_2', 'NAME_3'])
FLOATS = num.array([ 0.5    , 0.2     , 0.3     ])

DAT =  num.column_stack((NAMES, FLOATS))

I want to stack these two lists together and write them to a text file in the form of columns; therefore, I want to use numpy.savetxt (if possible) to do this.

num.savetxt('test.txt', DAT, delimiter=" ") 

When I do this, I get the following error:

>>> num.savetxt('test.txt', DAT, delimiter=" ") 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Python/2.7/site-packages/numpy-1.8.0.dev_9597b1f_20120920-py2.7-macosx-10.8-x86_64.egg/numpy/lib/npyio.py", line 1047, in savetxt
    fh.write(asbytes(format % tuple(row) + newline))
TypeError: float argument required, not numpy.string_

The ideal output file would look like:

NAME_1    0.5
NAME_2    0.2
NAME_3    0.3

How can I write both strings and float numbers to a text file, possibly avoiding using csv ( I want to make if readable for other people )? Is there another way of doing this instead of using numpy.savetxt?

Nectar

Answer 1

You have to specify the format (fmt) of you data in savetxt, in this case as a string (%s):

num.savetxt('test.txt', DAT, delimiter=" ", fmt="%s") 

The default format is a float, that is the reason it was expecting a float instead of a string and explains the error message.

Poock

And what happens if I want to format the numbers? How can I give some format to the numbers only? – Forecourt

You can give different formats to different columns, like fmt="%f %i". But if you want to combine it with strings as in the example above, you will need a structured array (see here). In this example, the int is converted to a string, and then formatting as an int is not possible anymore. – Poock

Answer 2

The currently accepted answer does not actually address the question, which asks how to save lists that contain both strings and float numbers. For completeness I provide a fully working example, which is based, with some modifications, on the link given in @joris comment.

import numpy as np

names  = np.array(['NAME_1', 'NAME_2', 'NAME_3'])
floats = np.array([ 0.1234 ,  0.5678 ,  0.9123 ])

ab = np.zeros(names.size, dtype=[('var1', 'U6'), ('var2', float)])
ab['var1'] = names
ab['var2'] = floats

np.savetxt('test.txt', ab, fmt="%10s %10.3f")

Update: This example also works properly in Python 3 by using the 'U6' Unicode string dtype, when creating the ab structured array, instead of the 'S6' byte string. The latter dtype would work in Python 2.7, but would write strings like b'NAME_1' in Python 3.

Poultry

What if I had a bidimensional floats array and wanted to print each name with its correspondent array? Would the only way be adding all the rows manually or is there a simpler way to do that? Expected output: NAME_1 0.1 0.2 0.3 – Kopeisk

Answer 3

import numpy as np

a = 'a,b,c'

b = [[1, 2, 3], [4, 5, 6]]

with open('test.csv', 'w') as f:
    f.writelines(a + '\n')
    np.savetxt(f, b, delimiter=',', fmt='%f')



The output file would look like this:
a,b,c
1.000000,2.000000,3.000000
4.000000,5.000000,6.000000

This example is for numpy.savetxt to write strings and float numbers to file, especially for strings that are written in rows.

Kronfeld

Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center. – Connective

Answer 4

There is actually a header parameter:

np.savetxt(file, data, delimiter=',', header='TimeStamp,X,Z', fmt='%s,%.2f,%d')
    

Inessential

Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center. – Connective