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: 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.]]
    

