Question

I have a list of objects (clusters) and each object has an attribute vertices which is a list of numbers. I want to construct a dictionary (using a one liner) such that the key is a vertex number and the value is the index of the corresponding cluster in the actual list.

Ex:

clusters[0].vertices = [1,2]
clusters[1].vertices = [3,4]

Expected Output:

{1:0,2:0,3:1,4:1}

I came up with the following:

dict(reduce(lambda x,y:x.extend(y) or x, [
     dict(zip(vertices, [index]*len(vertices))).items()
     for index,vertices in enumerate([i.vertices for i in clusters])]))

It works… but is there a better way of doing this?

Also comment on the efficiency of the above piece of code.

PS: The vertex lists are disjoint.

Answer 1

This is a fairly simple solution, using a nested for:

dict((vert, i) for (i, cl) in enumerate(clusters) for vert in cl.vertices)

This is also more efficient than the version in the question, since it doesn’t build lots of intermediate lists while collecting the data for the dict.