[Numpy-discussion] Reordering 2 dimensional array by column

Nicole Stoffels nicole.stoffels@forwind...
Thu Aug 2 08:08:27 CDT 2012


Thanks eat! It helps and it's so easy! :)

On 02.08.2012 14:59, eat wrote:
> Hi,
>
> On Thu, Aug 2, 2012 at 3:43 PM, Nicole Stoffels 
> <nicole.stoffels@forwind.de <mailto:nicole.stoffels@forwind.de>> wrote:
>
>     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]])
>
> Perhaps along the lines:
> In []: a
> Out[]:
> array([[1, 2, 3],
>        [0, 2, 1],
>        [5, 7, 8]])
> In []: ndx= a[:, 2].argsort()
> In []: a[ndx[::-1], :]
> Out[]:
> 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!
>
> My 2 cents,
> -eat
>
>
>     Best regards,
>
>     Nicole
>
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>
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