List Match In Python: Get Indices Of A Sub-list In A Larger List
For two lists, a = [1, 2, 9, 3, 8, ...] (no duplicate values in a, but a is very big) b = [1, 9, 1,...] (set(b) is a subset of set(a), 1<
Solution 1:
A fast method (when a is a large list) would be using a dict to map values in a to indices:
>>>index_dict = dict((value, idx) for idx,value inenumerate(a))>>>[index_dict[x] for x in b]
[0, 2, 0]
This will take linear time in the average case, compared to using a.index which would take quadratic time.
Solution 2:
Presuming we are working with smaller lists, this is as easy as:
>>>a = [1, 2, 9, 3, 8] >>>b = [1, 9, 1] >>>[a.index(item) for item in b]
[0, 2, 0]
On larger lists, this will become quite expensive.
(If there are duplicates, the first occurrence will always be the one referenced in the resulting list, if not set(b) <= set(a), you will get a ValueError).
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