[SciPy-user] Ordering and Counting the Repetitions of the Rows of a Matrix

Lorenzo Isella lorenzo.isella@gmail....
Sun Jul 27 11:17:47 CDT 2008


Dear All,
Consider an Nx2 matrix of the kind:

A=   1 2
       3 13
       1  2
       6  8
       3 13
       2  9
       1  1


The first entry in each row is always smaller or equal than the second
entry in the same row.
Now there are two things I would like to do with this A matrix:
(1) With a sort of n.unique1d (but have not been very successful yet),
let each row of A appear only once (i.e. get rid of the repetitions).
Therefore one should obtain the matrix:
B=   1 2
       3 13
       6  8
       2  9
       1  1

(2) At the same time, efficiently count how many times each row of B
appeared in A. I would like to get a C vector counting them as:

C=   2
       2
       1
       1
       1


Any suggestions about an efficient way of achieving this?
Many thanks

Lorenzo


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