Select cells randomly from NumPy array - without replacement

Question

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 replacemen

Answer 1

Use random.sample to generates ints in 0 .. A.size with no duplicates, then split them to index pairs:

import random
import numpy as np

def randint2_nodup( nsample, A ):
    """ uniform int pairs, no dups:
        r = randint2_nodup( nsample, A )
        A[r]
        for jk in zip(*r):
            ... A[jk]
    """
    assert A.ndim == 2
    sample = np.array( random.sample( xrange( A.size ), nsample ))  # nodup ints
    return sample // A.shape[1], sample % A.shape[1]  # pairs


if __name__ == "__main__":
    import sys

    nsample = 8
    ncol = 5
    exec "\n".join( sys.argv[1:] )  # run this.py N= ...
    A = np.arange( 0, 2*ncol ).reshape((2,ncol))

    r = randint2_nodup( nsample, A )
    print "r:", r
    print "A[r]:", A[r]
    for jk in zip(*r):
        print jk, A[jk]