best way to determine sequence ordering?

[Kay Schluehr]

* building a dict of indicies::

positions = dict((item, i) for i, item in enumerate(L))

if positions['A'] < positions['D']:
# do some stuff

You'll only get a gain from this version if you need to do several
comparisons instead of just one.

Hi Steven,

your solution may not create the correct answer if an item occurs twice
in the list because the later occurrence overwrites the former during
dict creation:

L = ['C', 'A', 'D', 'B', 'A']
dict((item, i) for i, item in enumerate(L))

{'A': 4, 'C': 0, 'B': 3, 'D': 2}

This gives the impression that 'D' always precedes 'A' which is wrong.

 

[Steven Bethard]

* building a dict of indicies::

positions = dict((item, i) for i, item in enumerate(L))

if positions['A'] < positions['D']:
# do some stuff

You'll only get a gain from this version if you need to do several
comparisons instead of just one.

Hi Steven,

your solution may not create the correct answer if an item occurs twice
in the list because the later occurrence overwrites the former during
dict creation:

L = ['C', 'A', 'D', 'B', 'A']
dict((item, i) for i, item in enumerate(L))

{'A': 4, 'C': 0, 'B': 3, 'D': 2}

This gives the impression that 'D' always precedes 'A' which is wrong.

Yeah, thanks for the update. I meant to include that too.

STeVe