[Numpy-discussion] Reordering 2 dimensional array by column

Nicole Stoffels nicole.stoffels@forwind...
Thu Aug 2 07:43:31 CDT 2012


Dear all,

I have a two-dimensional array:

a = array([[1,2,3],[0,2,1],[5,7,8]])

I want to reorder it by the last column in descending order, so that I get:

b =array([[5, 7, 8],[1, 2, 3],[0, 2, 1]])

What I did first is the following, which reorders the array in ascending 
order (I found that method in the internet):

b = array(sorted(a, key=lambda new_entry: new_entry[2]))
b = array([[0, 2, 1],[1, 2, 3],[5, 7, 8]])

But I want it just the other way arround. So I did the following 
afterwards which results in an array only containing zeros:
b_indices = b.argsort()
b_matrix = b[b_indices[::-1]]
new_b = b_matrix[len(b_matrix)-1]

Is there an easy way to reorder it? Or is there at least a complicated 
way which produces the right output?

I hope you can help me! Thanks!

Best regards,

Nicole



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