[Numpy-discussion] Re: selecting random array element

Robert Kern robert.kern at gmail.com
Mon Feb 20 15:47:01 CST 2006

Chris Fonnesbeck wrote:
> What is the best way to select a random element from a numpy array? I
> know I could index by a random integer, but was wondering if there was
> a built-in method or function.

Generating a random index is what I do.

I think there's certainly room for a RandomState.choice() method. I think
something like this covers most of the use cases:

import numpy
from numpy import random
def choice(x, axis=None):
    """Select an element or subarray uniformly randomly.
    If axis is None, then a single element is chosen from the entire array.
    Otherwise, a subarray is chosen from the given axis.
    x = numpy.asarray(x)
    if axis is None:
        length = numpy.multiply.reduce(x.shape)
        n = random.randint(length)
        return x.flat[n]
        n = random.randint(x.shape[axis])
        # I'm sure there's a better way of doing this
        idx = map(slice, x.shape)
        idx[axis] = n
        return x[tuple(idx)]

Robert Kern
robert.kern at gmail.com

"In the fields of hell where the grass grows high
 Are the graves of dreams allowed to die."
  -- Richard Harter

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