python - Create set and lists with the positions in the set efficently -

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i need create set of ids of messages, , positions in original list. code used sort messages later handle them according id.

the following works, readable, slow. 

import numpy np ids=np.array([354,45,45,34,354])#example, actual array huge  dict={} counter in xrange(len(ids)):     try:         dict[ids[counter]].append(counter)     except:         dict[ids[counter]]=[counter] print(dict) #{354: [0, 4], 34: [3], 45: [1, 2]}

any ideas how speed up? there no need lists sorted. later in code used follows, , after dict discarded

for item in dict.values():     position_of_id=position[np.array(item)]     ...

try use defaultdict , enumerate:

from collections import defaultdict     dict = defaultdict(list) i,id in enumerate(ids):     dict[id].append(i)

(using try , except bad idea if exceptions aren't rare)


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