fastest method to choose a random element

[Paul Hankin]

On Jan 5, 4:14 pm, c... at mailinator.com wrote:
> On Jan 5, 5:07 pm, c... at mailinator.com wrote:
>
> > Hello, Paul and Arnaud.
> > While I think about your answers: do you think there is any way to
> > avoid shuffle?
> > It may take unnecessary long on a long list most of whose elements
> > have the property.

You could generate a random shuffle of range(len(seq)) lazily, and use
that to iterate over your sequence.

import random
import itertools

def randxrange(n):
    "Generate the numbers 0 to n-1 in a random order"
    x = range(n)
    for i in xrange(n):
        r = random.randrange(n - i)
        yield x[r]
        x[r] = x[n - i - 1]

def shuffled(seq):
    "Generate the elements of seq in a random order"
    return (seq[i] for i in randxrange(len(seq)))

def pick_random(seq, prop):
    return itertools.ifilter(prop, shuffled(seq)).next()

--
Paul Hankin