amine23 amine23 - 1 year ago 172
Python Question

Implementation of numpy in1d for 2D arrays?

I have a 2D numpy array S representing a state space, with 80000000 rows (as states) and 5 columns (as state variables).

I initialize K0 with S, and at each iteration, I apply a state transition function f(x) on all of the states in Ki, and delete states whose f(x) is not in Ki, resulting Ki+1. Until it converges i.e. Ki+1 = Ki.

Going like this would take ages:

K = S
to_delete = [0]
While to_delete:
to_delete = []
for i in xrange(len(K)):
if not f(i) in K:
K = delete(K,to_delete,0)

So I wanted to make a vectorized implementation :

slice K in columns, apply f and, join them once again, thus obtaining f(K) somehow.

The question now is how to get an array of length len(K), say Sel, where each row Sel[i] determine whether f(K[i]) is in K. Exactly like the function in1d works.

Then it would be simple to make


Answer Source

Your question is difficult to understand because it contains extraneous information and contains typos. If I understand correctly, you simply want an efficient way to perform a set operation on the rows of a 2D array (in this case the intersection of the rows of K and f(K)).

You can do this with numpy.in1d if you create structured array view.


if this is K:

In [50]: k
array([[6, 6],
       [3, 7],
       [7, 5],
       [7, 3],
       [1, 3],
       [1, 5],
       [7, 6],
       [3, 8],
       [6, 1],
       [6, 0]])

and this is f(K) (for this example I subtract 1 from the first col and add 1 to the second):

In [51]: k2
array([[5, 7],
       [2, 8],
       [6, 6],
       [6, 4],
       [0, 4],
       [0, 6],
       [6, 7],
       [2, 9],
       [5, 2],
       [5, 1]])

then you can find all rows in K also found in f(K) by doing something this:

In [55]: k[np.in1d(k.view(dtype='i,i').reshape(k.shape[0]),k2.view(dtype='i,i').
Out[55]: array([[6, 6]])

view and reshape create flat structured views so that each row appears as a single element to in1d. in1d creates a boolean index of k of matched items which is used to fancy index k and return the filtered array.

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