Re: inverting a dictionary of lists

[Gabriel Genellina]

En Fri, 15 Jun 2007 00:20:33 -0300, [EMAIL PROTECTED] escribió:

 I searched for an hour and don't see a solution to this (i assume
 somewhat common) problem.

 I have a very large dictionary of lists:
 d = {a:[1,2], b:[2,3], c:[3]}
 and i want to reverse the associativity of the integers thusly:
 inverse(d)   makes   {1:[a], 2:[a,b], 3:[b,c]}

 my solution expands the original dict into two lists of keys and list
 elements:
 list1:  [a,a,b,b,c]
 list2:  [1,2,2,3,3]
 then recombines them with the reverse operation.

 but this takes too much time and a lot of memory.
 I wonder if anyone can point me to a more efficient solution?

py d = dict(a=[1,2], b=[2,3], c=[3])
py result = {}
py for k,v in d.iteritems():
...   for item in v:
... result.setdefault(item, []).append(k)
...
py result
{1: ['a'], 2: ['a', 'b'], 3: ['c', 'b']}
py

You may use collections.defaultdict too - search some recent posts.

-- 
Gabriel Genellina

-- 
http://mail.python.org/mailman/listinfo/python-list

Re: inverting a dictionary of lists

[Raymond Hettinger]

On Jun 14, 8:20 pm, [EMAIL PROTECTED] wrote:
 I have a very large dictionary of lists:
 d = {a:[1,2], b:[2,3], c:[3]}
 and i want to reverse the associativity of the integers thusly:
 inverse(d)   makes   {1:[a], 2:[a,b], 3:[b,c]}



Try using setdefault:

 d = {'a':[1,2], 'b':[2,3], 'c':[3]}
 r = {}
 for k in d:
for e in d[k]:
r.setdefault(e, []).append(k)

 r
{1: ['a'], 2: ['a', 'b'], 3: ['c', 'b']}


Raymond

-- 
http://mail.python.org/mailman/listinfo/python-list