Finding indices of multiple occurring items in a list

Question

I'm trying to accumulate a list of indices of items that occur multiple times, given a list. Not sure how to go about doing that as my code only manages to compare pattern[1] and pattern[2] before terminating.

def test(pattern):
     """(list) -> list of int

     >>> test(['A', 'B', 'A', 'C', 'A'])
     [0, 2, 4]
     >>> test(['A', 'B'])
     []
     """
     indices = []
     new_list = []

     for i in range(len(pattern) - 1): 
          if pattern[i][-1] == pattern[i + 1]:  
              indices.append(i)
              new_list = phoneme_list[max(indices):]

      return new_list

How long are the lists you're working with?

TigerhawkT3
– TigerhawkT3

2015-04-01 23:56:19 +00:00
Commented Apr 1, 2015 at 23:56 — TigerhawkT3
– TigerhawkT3, Commented Apr 1, 2015 at 23:56

TigerhawkT3 · Accepted Answer · 2015-04-01 23:35:04Z

3

>>> lst = ['A', 'B', 'A', 'C', 'A']
>>> [i for i in range(len(lst)) if lst.count(lst[i]) > 1]
[0, 2, 4]

That said, assembling a list of indices probably indicates that your algorithm could be improved.

answered Apr 1, 2015 at 23:35

TigerhawkT3

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Comments

Adam Smith · Accepted Answer · 2015-04-01 23:53:42Z

I'd do this:

import collections

d = collections.defaultdict(list)

for idx,el in enumerate(lst):
    d[el].append(idx)
    # you can also do this with d as a normal dict, and instead do
    # d.setdefault(el, []).append(idx)

# this builds {'A':[0,1,3], 'B':[2,4], 'C':[5]} from
# ['A','A','B','A','B','C']

result = [idx for idxs in d.values() for idx in idxs if len(idxs) > 1]
# this builds [0,1,3,2,4] from
# {'A':[0,1,3], 'B':[2,4], 'C':[5]}

It also avoids the need to call list.count n times, which should perform massively faster for a larger dataset.

Alternatively you could leverage collections.Counter to get all the values that happen multiple times, then pull all their indices at once.

import collections

c = set([el for el,val in collections.Counter(lst).items() if val>1])
# gives {'A','B'}
result = [idx for idx,el in enumerate(lst) if el in c]
# gives [1,2,3,4]

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