[Tutor] Permutations? (original) (raw)

Hee-Seng Kye kyeser at earthlink.net
Sun Jul 25 09:40:00 CEST 2004

def perm(k): # Compute the list of all permutations of k if len(k) <= 1: return [k] r = [] for i in range(len(k)): s = k[:i] + k[i+1:] p = perm(s) for x in p: r.append(k[i:i+1] + x) return r

Could someone tell me how I can modify the above function so that it produces a list of permutations of k that only begins on k[0]?

If k = [0,1,2,3], I want to modify perm(k) so that it only produces [[0,1,2,3], [0,1,3,2], [0,2,1,3], [0,2,3,1], [0,3,1,2], [0,3,2,1]].

I could produce the list of ALL permutations of k and then delete the ones which don't begin with k[0], but for a large list, this could really slow things down. I was thinking of algorithm that computes only the ones I want.

I would appreciate anyone's suggestion. Thanks.

Best, Kye

p.s. By the way, does anyone have any idea if it would drive my computer (667 MHz and 512MB RAM) nuts to perm(range(12))?

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