[Numpy-discussion] Funky vectorisation question

[David Warde-Farley]

On 29-Apr-09, at 5:06 PM, Dan Goodman wrote:

> Here's the problem I want to write vectorised code for. I start with  
> an
> array of indices, say I=array([0,1,0,2,0,1,4]), and I want to come up
> with an array C that counts how many times each index has been seen so
> far if you were counting through the array from the beginning to the
> end, e.g. for that I, C=array([0,0,1,0,2,1,0]). This is easy enough to
> do with a C loop, but trickier to do just with numpy. Any ideas?

How about this:

In [28]: I = array([0,1,0,2,0,1,4])

In [29]: C = empty(len(I),dtype=int)

In [30]: for idx in unique(I):
    ....:     V = I == idx
    ....:     C[V] = arange(len(V))
    ....:

In [31]: C
Out[31]: array([0, 0, 1, 0, 2, 1, 0])

David