I have a very large igraph object so that calculation of shortest paths takes a long time.
I am only interested in the length of the shortest path between a very small set of pairs of vertices.
Let's say the (undirected) graph consists of 10,000 vertices and has 500,000 edges, there are shortest paths for 10,000 * 10,000 / 2 pairs of vertices, but I only need the paths between 10,000 pairs of vertices.
Is there any possibility to define not only vertices, but pairs of vertices (meaning: start and end point of the path to be calculated)?
Since you have even number of vertices to make pairs, you can divide all vertices into two groups, i.e., even or odd, like below
v_even <- subset(V(g), !V(g) %% 2)
v_odd <- subset(V(g), !!V(g) %% 2)
and then you run shortest_paht with mapply to produce the shortest paths
> mapply(function(x, y) shortest_paths(g, x, y)$vpath, v_even, v_odd)
[[1]]
+ 3/10 vertices, from 7125d30:
[1] 2 6 1
[[2]]
+ 2/10 vertices, from 7125d30:
[1] 4 3
[[3]]
+ 2/10 vertices, from 7125d30:
[1] 6 5
[[4]]
+ 5/10 vertices, from 7125d30:
[1] 8 5 6 2 7
[[5]]
+ 3/10 vertice
Data
set.seed(1)
g <- sample_gnm(10, 15)