[Numpy-discussion] Inverted indices

[Nicolas P. Rougier]

Nice ! Thanks Stéfan.
I will add it to the numpy 100 problems.


Nicolas



On 07 Aug 2014, at 13:31, Stéfan van der Walt <stefan at sun.ac.za> wrote:

> Hi Nicolas
> 
> On Thu, Aug 7, 2014 at 1:16 PM, Nicolas P. Rougier
> <Nicolas.Rougier at inria.fr> wrote:
>> Here is a small example:
>> 
>> Z = [(0,0), (1,1), (2,2), (3,3), (4,4))
>> I  = [0, 20, 23, 24, 37]
>> 
>> S = [ 20,20,0,24]
>> -> Result should be [(1,1), (1,1), (0,0),(3,3)]
>> 
>> S = [15,15]
>> -> Wrong (15 not in I) but ideally, I would like this to be converted to [(0,0), (0,0)]
> 
> First try:
> 
> Z = np.array([(0,0), (1,1), (2,2), (3,3), (4,4)])
> I = np.array([0, 20, 23, 24, 37])
> S = np.array([ 20,20,0,24,15])
> 
> out = np.zeros((len(S), len(Z[0])))
> mask = (S[:, np.newaxis] == I)
> item, coord = np.where(mask)
> out[item, :] = Z[coord]
> 
> Perhaps there's a neater way of doing it!
> 
> Stéfan
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