Recipe 578227 revision 1

def perm_parity(lst):
    '''\
    Given a permutation of the digits 0..N in order as a list, 
    returns its parity (or sign): +1 for even parity; -1 for odd.
    '''
    parity = 1
    for i in range(0,len(lst)-1):
        if lst[i] != i:
            parity *= -1
            mn = min(range(i,len(lst)), key=lst.__getitem__)
            lst[i],lst[mn] = lst[mn],lst[i]
    return parity    

if __name__ == '__main__':
    from itertools import permutations

    for p in permutations(range(3)):
        l = list(p)
        print "%2i %r" % (perm_parity(l), p)

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• revision 1 (11 years ago)