[SciPy-User] sorting an array

[Martin]

Hi all,

Thanks for the replies...perhaps if I explained the context it might
help (or it might not!). Basically it is an image processing problem
so the array is rows, columns. So I want to sort the array such that

row col
1   1
1   2
1   3
etc
2   1
2   2
2   3

then I can use the other columns to create images safe in the
knowledge they are in the correct row, column 2D space. I think
Emmanuelle solution does the trick from what I can see, I am just
trying it out. Although I have never seen [:,:2] what does the :2 bit
mean?

Neil your solution results in column 4 changing some of the ordering
from what I can see (the zero in the fourth column).

Thanks a lot for the suggestions

Martin



On Aug 1, 11:26 pm, Neil Crighton <neilcrigh... at gmail.com> wrote:
> Martin <mdekauwe <at> gmail.com> writes:
>
> > When really what i wanted was not for the 3rd and 4th columns to also
> > be sorted nunmerically in that way. In unix I would reformat the first
> > two columns in gawk i.e. gawk '{printf("%2.3d %2.3d, %d, %d\n", $1,
> > $2, $3, $4)}' | sort -n -k 1
>
> I think lexsort does what you need. Get the sorted row indices:
>
> >>> a
>
> array([[ 1,  0,  2,  5],
>        [ 1,  5,  6,  2],
>        [ 1,  2,  8,  7],
>        [ 1,  4,  4,  6],
>        [ 1,  3,  2,  3],
>        [ 1, 11,  2,  0],
>        [ 1, 10,  1,  3],
>        [ 1,  9,  0,  4],
>        [ 1,  8,  9,  6]])
>
> >>> ind = np.lexsort(a.T[::-1])
>
> a.T gives an array of columns, and reversing them with ::-1 sorts by the
> first column values, then the second column values if the first column
> values are equal, and so on. Now index the the original array:
>
> >>> ind
>
> array([0, 2, 4, 3, 1, 8, 7, 6, 5])>>> a[ind]
>
> array([[ 1,  0,  2,  5],
>        [ 1,  2,  8,  7],
>        [ 1,  3,  2,  3],
>        [ 1,  4,  4,  6],
>        [ 1,  5,  6,  2],
>        [ 1,  8,  9,  6],
>        [ 1,  9,  0,  4],
>        [ 1, 10,  1,  3],
>        [ 1, 11,  2,  0]])
>
> Neil
>
> _______________________________________________
> SciPy-User mailing list
> SciPy-U... at scipy.orghttp://mail.scipy.org/mailman/listinfo/scipy-user