John Salerno wrote:
> If I want to make a list of four items, e.g. L = ['C', 'A', 'D', 'B'],
> and then figure out if a certain element precedes another element, what
> would be the best way to do that?
>
> Looking at the built-in list functions, I thought I could do something
> like:
>
> if L.index('A') < L.index('D'):
> # do some stuff
This is probably a pretty reasonable approach as long as you're not too
worried about efficiency. It scans the list twice though, so if you
start doing this with really big lists, it might be better to do
something like:
>>> L = ['C', 'A', 'D', 'B']
>>> positions = dict((item, i) for i, item in enumerate(L))
>>> positions
{'A': 1, 'C': 0, 'B': 3, 'D': 2}
>>> positions['A'] < positions['D']
True
If you care about memory in addition to speed, you could do something like:
>>> L = ['C', 'A', 'D', 'B']
>>> items_to_compare = ['A', 'D']
>>> positions = dict(
... (item, i)
... for i, item in enumerate(L)
... if item in items_to_compare
... )
>>> positions
{'A': 1, 'D': 2}
>>> positions['A'] < positions['D']
True
STeVe
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