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

Travis Oliphant oliphant.travis at ieee.org
Thu Jun 1 19:32:03 EDT 2006

Tom Denniston 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.

I've just changed lexsort to accept any sequence object as keys. This means that it can now be used to sort a 2d array (of the same data-type) based on multiple rows. The sorting will be so that the last row is sorted with any repeats sorted by the second-to-last row and remaining repeats sorted by the third-to-last row and so forth...

The return value is an array of indices. For the 2d example you can use

ind = lexsort(a) sorted = a[:,ind] # or a.take(ind,axis=-1)

Example:

a = array([[1,3,2,2,3,3],[4,5,4,6,4,3]]) ind = lexsort(a) sorted = a.take(ind,axis=-1) sorted array([[3, 1, 2, 3, 3, 2], [3, 4, 4, 4, 5, 6]]) a array([[1, 3, 2, 2, 3, 3], [4, 5, 4, 6, 4, 3]])

-Travis

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