Glazner wrote:
>> > def invert(p):
>> > inverse = [None] * len(p)
>> > for (i, j) in enumerate(p):
>> > inverse[j] = i
>> > return inverse
>>
>> Elegant. This seems like the best solution, although it isn't as much
>> fun to write as a "one-liner". Thanks
>
>
>>>> invert([1, 2, 3, 1])
> [None, 3, 1, 2] #blah
1 occurs twice in [1, 2, 3, 1] which therefore doesn't describe a
permutation. In general a function has to be "bijective" to be invertable.
You can catch the problem with (untested)
def invert(p):
inverse = [None] * len(p)
for i, k in enumerate(p):
if inverse[k] is not None:
raise ValueError
inverse[k] = i
return inverse
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