numpy.savetxt "tuple index out of range"?

Question

I'm trying to write a few lines into a text file, and here's the code I used:

import numpy as np

# Generate some test data
data = np.arange(0.0,1000.0,50.0)

with file('test.txt', 'w') as outfile:      
    outfile.write('# something')

    for data_slice in data: 
        np.savetxt(outfile, data_slice, fmt='%1.4e')

        outfile.write('# New slice\n')

When the code runs up to the line with savetxt, I get this error:

     IndexError: tuple index out of range

Any idea why this happens? I tried removing the "fmt" part, but I get the same thing.

Answer 1

the problem is that numpy.save expect an array with some shape information, while you pass it just a number.

if you want to pass one element at the time (but I suggest you to save the whole array) you have to convert it first to a numpy array with a shape of at least one

np.savetxt(outfile, array(data_slice).reshape(1,), fmt='%1.4e')

this is because the shape of a single number is a void tuple, and to write to file it try to split along the first dimension

array(1).shape == tuple()
#True

to save the whole array it is sufficient to do:

np.savetxt(outfile, data, fmt='%1.4e')