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@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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