如何使用R igraph访问给定最短路径中的访问顶点

Question

I use all_shortest_paths function to identify all the shortest paths for given two vertices.

I do not know the differences, but the following two functions give me the same results

all_shortest_paths(g, 1,3)
get.all.shortest.paths(g, 1,3)

Here is the outcome

$res
$res[[1]]
+ 3/9 vertices, from a86e634:
[1] 1 4 3

$res[[2]]
+ 3/9 vertices, from a86e634:
[1] 1 2 3


$nrgeo
[1] 1 1 2 1 1 0 1 1 1

Now, I want to get the nodes that are visited in a path without the source and sink nodes. For instance, I get the first shortest path.

> all_shortest_paths(g, 1,3)$res[1]
[[1]]
+ 3/9 vertices, from a86e634:
[1] 1 4 3`

How can I store the nodes that are visited excluding the source and sink nodes (ie, 1,3)? When I assign a<- all_shortest_paths(g, 1,3)$res[1] , its type seems like list, but no matter what I am doing, I cannot access 4. It keeps returning me + 3/9 vertices, from a86e634: [1] 1 4 3

Answer 1

You need to go one more level down the list returned by all_shortest_paths . In the code below I create the variable n to make it more readable.

library(igraph)

g <- make_graph("Cubical")

p <- all_shortest_paths(g, 1, 3)

n <- length(p[[1]][[1]])
p[[1]][[1]][-c(1, n)]
#+ 1/8 vertex, from 0de75ff:
#[1] 4

To get all inner vertices in one go, use lapply on p[[1]] .

lapply(p[[1]], function(.p){
  n <- length(.p)
  .p[-c(1, n)]
})
#[[1]]
#+ 1/8 vertex, from 0de75ff:
#[1] 4
#
#[[2]]
#+ 1/8 vertex, from 0de75ff:
#[1] 2

This code does not depend on the number of inner vertices, as can be seen if the source and sink are 1 and 7.
This time a one-liner.
(Output omitted.)

p2 <- all_shortest_paths(g, 1, 7)

lapply(p2[[1]], function(.p) .p[-c(1, length(.p))])