# Cluster

#### John Coene

#### 2019-04-09

Since `v3.0.0` you can color the nodes by cluster. Simply use `sg_cluster` with your igraph clustering algorithm of choice.

    nodes <- sg_make_nodes()
    edges <- sg_make_edges(nodes, 17)

    sigmajs() %>%
      sg_nodes(nodes, id, size) %>%
      sg_edges(edges, id, source, target) %>%
      sg_layout() %>%
      sg_cluster()

Clustering is brilliant, in part, because (most) are bottom-up approaches; the number of cluster is not pre-defined. However, this might make the above approach somewhat awkward as the node colors should be discrete. In an interactive environment `sg_cluster` prints the number of clusters identified to the console, so you could then set the exact number of colors needed then re-run the graph.

Perhaps a better approach is to use the helper function `sg_get_cluster` to return nodes with an added column, the group they belong to. The helper also lets you set (or re-set) the node colors according to the group they belong to.

Let’s take a look at an example

    set.seed(19880525) # for reproducability

    # make a slightly larger graph
    nodes <- sg_make_nodes(50)
    edges <- sg_make_edges(nodes, 100)

    # use the helper
    # first run showed 11 clusters
    nodes <- sg_get_cluster(
      nodes, edges, colors = c(
        "#331A00", "#663000",
        "#996136", "#CC9B7A",
        "#D9AF98", "#F2DACE",
        "#CCFDFF", "#99F8FF",
        "#66F0FF", "#33E4FF",
        "#00AACC")
      )

    sigmajs() %>%
      sg_nodes(nodes, id, size, color) %>%
      sg_edges(edges, id, source, target) %>%
      sg_layout() %>%
      sg_neighbours()