Generating an inverse index

Question

I have the following:

strlist = ['the', 'the', 'boy', 'happy', 'boy', 'happy']
{x:{(list(enumerate(strlist))[y])[0]} for y in range(len(strlist)) for x in (strlist)}

My output is the following:

{'boy': set([5]), 'the': set([5]), 'happy': set([5])}

My issue is that I'd like to output this (using python 3.x):

{'boy': {2,4}, 'the': {0,1}, 'happy': {3,5} }

Any help would be great!

Thanks

Answer 1

>>> strlist = ['the', 'the', 'boy', 'happy', 'boy', 'happy']
>>> from collections import defaultdict
>>> D = defaultdict(set)
>>> for i, s in enumerate(strlist):
...     D[s].add(i)
... 
>>> D
defaultdict(<type 'set'>, {'boy': {2, 4}, 'the': {0, 1}, 'happy': {3, 5}})

If you can't use defaultdict for some reason

>>> D = {}
>>> for i, s in enumerate(strlist):
...     D.setdefault(s, set()).add(i)
... 
>>> D
{'boy': {2, 4}, 'the': {0, 1}, 'happy': {3, 5{}

Here is the silly (inefficient) way to write it as a comprehension

>>> {k: {i for i, j in enumerate(strlist) if j == k} for k in set(strlist)}
{'boy': {2, 4}, 'the': {0, 1}, 'happy': {3, 5}}

Answer 2

Try

dict(((string, set(i for i,w in enumerate(strlist) if w == string)) for string in strlist))

but note it has quadratic runtime, so it is only useful for very small amounts of data.

Test case and sample output: http://ideone.com/4sxUNf