R中的igraph：get.shortest.paths错误，但shortsts.paths错误

Question

I have encountered strange behavior using igraph in R. shortest.paths command returns correct results, get.shortest.paths returns warning and no results.

shortest.paths(g, v=2795, to=2839) # correct

         [,1]
    [1,] 3930.4

get.shortest.paths(g, from=2795, to=2839) # warning and no results

    [[1]]

    numeric(0)

Warning message:
In get.shortest.paths(g_novy, from = 2795, to = 2839) :
  At structural_properties.c:5296 :Couldn't reach some vertices

Does anyone know, whats the problem?

Thanks, Zbynek

Answer 1

My guess is that you have a directed graph. The shortest.paths function will tell you the length of the shortest undirected path. The get.shortest.paths function tells you that there is no directed path between the vertices. Below is the simplest example of what seems to be happening:

g <- graph(1:2)
plot(g)
shortest.paths(g, v=2, to=1)
#       [,1]
#  [1,]    1
get.shortest.paths(g, from=2, to=1)
#  [[1]]
#  numeric(0)
#  
#  Warning message:
#  In get.shortest.paths(g, from = 2, to = 1) :
#    At structural_properties.c:706 :Couldn't reach some vertices