Find node index of connected graph components using conncomp - matlab

Question

Problem
I am trying to find the connected components of my undirected graph.
Matlabs function conncomp does exactly this. Mathworks - connected graph components
Example
Using the example given on matlabs webpage to keep it easy and repeatable:
G = graph([1 1 4],[2 3 5],[1 1 1],6);
plot(G)
bins = conncomp(G)
bins =
1 1 1 2 2 3
Two Question´s to this
First Question: Using this how can I find the initial node index, so that
cluster1 = (1 2 3); (instead of ( 1 1 1))
cluster2= (4 5); (instead of (2 2))
Second Question:
I am working on a big dataset and I know many nodes are not connected, so is there a way to only display clusters that contain more than one value ?
Thanks for your help, I am majorly stuck here.

Answer 1

You can use splitapply for the first part, like so:
clusters = splitapply(#(x) {x}, 1:numnodes(G), bins)
This returns a cell array where each cell contains the indices of the nodes in a group. You can filter this down in the usual way using cellfun
discard = cellfun(#isscalar, clusters);
clusters(discard) = [];
(Note that splitapply is new in R2015b - but the OP is using graph, also new in R2015b, so it should be fine for them)

Answer 2

Actually the first part of the question can be answered very simply as Matlabs conncomp provides a tool for this:
bins=conncomp(G,'OutputForm','cell');
Creates a cell array that contains the clusters, with all node names in the cells.
For the second part of the question I guess there are several ways but this one could be used as well:
clusters= bins(cellfun(#numel,bins)>1);