Write multiple numpy arrays to file

Question

I know how to use numpy.savetxt to write an array to a file. How can I write multiple arrays to the same file?

Essentially I want to do math to a column of numbers, and then replace the old column with the modified numbers. I read the easiest way to do this is to write a new file completely, put the modified numbers in, and just 'copy and paste' the other numbers in the file.

Any help is appreciated.

Thanks!

Answer 1

Answering a very old post for my own use. I've used the following to write out two 1D arrays of same size as CSV.

import numpy as np

x = [1, 2, 3]
y = [4, 5, 6]
zipped = zip(x, y)
# >>> [(1, 4), (2, 5), (3, 6)]

# Save the array back to the file
np.savetxt('z.csv', zipped, fmt='%i,%i')

Answer 2

If you're wanting to write multiple arrays to a file for later use, Look into numpy.savez.

However, from your description, it sounds like you're wanting to do something with a particular column of a delimited text file.

In that case, just load the entire thing in and operate on just the column you need to.

E.g.

import numpy as np

data = np.loadtxt('test.txt')

# Multiply the 4th column by 5
data[:,3] *= 5

# Do something more complicated to the 2nd column
data[:,1] = np.cos(data[:,1])

# Save the array back to the file
np.savetxt('test.txt', data)