Just saw this, in-place permutation (of a Java array, in this case).

In Python, if you ignore the battery that's included (random.shuffle) you have this:

import random
l = range(0, 20) # array to permute

for j in range(1, len(l)):
   k = random.randint(0, j)
   l[j], l[k] = l[k], l[j]

print l

Cute, but kinda ugly. It should be possible to make this more Pythonic.

--titus

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Legacy Comments

Posted by Doug Napoleone on 2007-06-05 at 22:32.

for j in xrange(1, len(L)): L.insert(j, L.pop(random.randint(0, j))
Because the pop decreases the local sub-length, the insert (which
inserts before), will insert in the proper place.    Not sure if its
'more pythonic' or not.

Posted by Doug Napoleone on 2007-06-05 at 22:40.

On second thought, I am sure my code will cause some nasty thrashing.
'Never mind'.

Posted by John Montgomery on 2007-06-06 at 08:03.

I recently did changed from using random.shuffle() to this generator:
def rand_seq(size):    values=range(size)    for i in xrange(size):
j=i+int(random.random()*(size-i))
values[j],values[i]=values[i],values[j]    yield values[i]    So I
could walk through elements in a random order, but abandon the walk
part way through.  Helped a lot, as it you only get a few steps in you
save a lot of calls to random().  Of course not sure how pythonic it
is...  Did have quite a few attempts at writing it well, but in the
end this worked the best.

Posted by Paul Moore on 2007-06-06 at 08:53.

Not sure what you expect by "more Pythonic". For a Pythonic solution,
I'd use random.shuffle :-)    You could probably do something with
itertools, but you'd still end up coding a loop over the elements.

Posted by Jack Diederich on 2007-06-06 at 10:44.

Use random.shuffle!  Shuffling isn't a hard thing to do right but it
is a very easy thing to do wrong as the <a
href="http://programming.reddit.com/info/1w1z3/comments">Reddit
thread</a> about the linked post points out.

Posted by Titus Brown on 2007-06-06 at 11:39.

Paul, Jack -- so, what did the guy who wrote 'random.shuffle' use?
random.shuffle?    ;)    --titus

Posted by Benji York on 2007-06-06 at 13:31.

Related article: How We Learned to Cheat at Online Poker: A Study in
Software Security
(http://www.cigital.com/papers/download/developer_gambling.php)
They exploited a bad shuffling algorithm to know what everyone's cards
were and what cards would be dealt next.