igraph: find continuously connected vertices starting from a vertex igraph r

Question

input:

data.frame(from = c(NA, 2, 4, 5, 8, 8, 1),
           to   = c(1, 3, 7, 6, 9, 10, 11))


 from to
   NA  1
    2  3
    4  7
    5  6
    8  9
    8 10
    1 11

I want to know how to use igraph to select a starting vertex, and get all the continuously connected vertices.

example

select(2) -> c(2, 3)
select(4) -> c(4, 7)
select(1) -> c(1, 11)
select(8) -> c(8, 9, 10)

The idea is: I don't know the ending vertex, I just know the starting point and I want to return the continuous path starting from this point

Answer 1

You can use either ego or all_simple_paths to find the connected nodes, which give slightly different output, depending on your need.( ego gives the neighbours up to a specified depth in one string, and all_simple_paths separates the nodes by path direction)

#Your example

library(igraph)

g <- graph_from_data_frame(data.frame(from = c(NA, 2, 4, 5, 8, 8, 1),
           to   = c(1, 3, 7, 6, 9, 10, 11)))

 all_simple_paths(g, "2")
# [[1]]
# + 2/12 vertices, named:
# [1] 2 3

 ego(g, length(V(g)), "2")
# [[1]]
# + 2/12 vertices, named:
# [1] 2 3

All together:

ego(g, length(V(g)), as.character(c(1,2,4,8)))

or

lapply(as.character(c(1,2,4,8)) , function(x) all_simple_paths(g, from=x))