[Numpy-discussion] lexsort (original) (raw)

Charles R Harris charlesr.harris at gmail.com
Fri Jun 2 01:05:13 EDT 2006

Tom,

The list -- nee tuple, thanks Travis -- is the list of key sequences and each key sequence can be a column in a matrix. So for instance if you wanted to sort on a few columns of a matrix, say columns 2,1, and 0, in that order, and then rearrange the rows so the columns were ordered, you would do something like:

a = randint(0,2,(7,4)) a array([[0, 0, 0, 1], [0, 0, 1, 0], [1, 0, 0, 1], [0, 1, 0, 1], [1, 1, 1, 0], [0, 1, 1, 1], [0, 1, 0, 1]]) ind = lexsort((a[:,2],a[:,1],a[:,0])) sorted = a[ind] sorted array([[0, 0, 0, 1], [0, 0, 1, 0], [0, 1, 0, 1], [0, 1, 0, 1], [0, 1, 1, 1], [1, 0, 0, 1], [1, 1, 1, 0]])

Note that the last key defines the major order.

Chuck

On 6/1/06, Tom Denniston <tom.denniston at alum.dartmouth.org> wrote:

This function is really useful but it seems to only take tuples not ndarrays. This seems kinda strange. Does one have to convert the ndarray into a tuple to use it? This seems extremely inefficient. Is there an efficient way to argsort a 2d array based upon multiple columns if lexsort is not the correct way to do this? The only way I have found to do this is to construct a list of tuples and sort them using python's list sort. This is inefficient and convoluted so I was hoping lexsort would provide a simple solution. --Tom

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