converters option in numpy genfromtxt not accepting -ve indexing of columns

Question

I want to load only last few columns in a text file with some evaluation.

I used numpy.genfromtxt with the argument converters={-1:func,-2:func}

But it is not working. On the other hand if i give the forward indexing like converters={56:func,57:func} it works correctly.

Why doesn't converters argument support the python's backward indexing? Is there anyway to do this if i know only the indexing of column from the last?

Answer 1

Using numpy.loadtxt it works, and you can use the converters parameter to define your functions. Having a tmp.txt file with:

11,12,13,14,15,16,17,18,19
21,22,23,24,25,26,27,28,29
31,32,33,34,35,36,37,38,39
41,42,43,44,45,46,47,48,49
51,52,53,54,55,56,57,58,59

You can load the selected columns with (also chosing the order which you want them to be stacked):

import numpy as np
print np.loadtxt('tmp.txt',delimiter=',',usecols=(-2,-1))
#[[ 18.  19.]
# [ 28.  29.]
# [ 38.  39.]
# [ 48.  49.]
# [ 58.  59.]]
print np.loadtxt('tmp.txt',delimiter=',',usecols=(-1,-2),converters={-1: lambda x: float(x)+100})
#[[ 119.   18.]
# [ 129.   28.]
# [ 139.   38.]
# [ 149.   48.]
# [ 159.   58.]]