number generator

Nick Craig-Wood nick at craig-wood.com
Mon Mar 12 12:30:04 CET 2007


Paul Rubin <http> wrote:
>  The fencepost method still seems to be simplest:
> 
>      t = sorted(random.sample(xrange(1,50), 4))
>      print [(j-i) for i,j in zip([0]+t, t+[50])]

Mmm, nice.

Here is another effort which is easier to reason about the
distribution produced but not as efficient.

def real(N, M):
    while 1:
        t = [ random.random() for i in range(N) ]
        factor = M / sum(t)
        t = [ int(round(x * factor)) for x in t]
        if sum(t) == M:
            break
        print "again"
    assert len(t) == N
    assert sum(t) == M
    return t

It goes round the while loop on average 0.5 times.

If 0 isn't required then just test for it and go around the loop again
if found.  That of course skews the distribution in difficult to
calculate ways!

-- 
Nick Craig-Wood <nick at craig-wood.com> -- http://www.craig-wood.com/nick



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