Select cells randomly from NumPy array - without replacement

python random numpy shuffle sampling
robintw picture robintw · Oct 8, 2010 · Viewed 14.4k times · Source

I'm writing some modelling routines in NumPy that need to select cells randomly from a NumPy array and do some processing on them. All cells must be selected without replacement (as in, once a cell has been selected it can't be selected again, but all cells must be selected by the end).

I'm transitioning from IDL where I can find a nice way to do this, but I assume that NumPy has a nice way to do this too. What would you suggest?

Update: I should have stated that I'm trying to do this on 2D arrays, and therefore get a set of 2D indices back.

Answer

Wolph picture Wolph · Oct 8, 2010

How about using numpy.random.shuffle or numpy.random.permutation if you still need the original array?

If you need to change the array in-place than you can create an index array like this:

your_array = <some numpy array>
index_array = numpy.arange(your_array.size)
numpy.random.shuffle(index_array)

print your_array[index_array[:10]]

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