Question

I have a data.frame with 2 columns: Node A, Node B. Each entry in the frame implies an edge in a graph between node A and B.

There must be a nice one-liner to convert this data.frame into an adjacency list. Any hints?

Answer 1

Quick and dirty …

> edges <- data.frame(nodea=c(1,2,4,2,1), nodeb=c(1,2,3,4,5))

> adjlist <- by(edges, edges$nodea, function(x) x$nodeb)

> for (i in as.character(unique(edges$nodea))) {
+   cat(i, ' -> ', adjlist[[i]], '\n')
+ }

1  ->  1 5
2  ->  2 4
4  ->  3

> adjlist
edges$nodea: 1
[1] 1 5
------------------------------------------------------------
edges$nodea: 2
[1] 2 4
------------------------------------------------------------
edges$nodea: 4
[1] 3