r - How to extract edge attributes from a directed graph shortest path:

Question

I am new to graphs and expect this is something fairly basic.

I have a directed graph (a hydrology network) and I want to grab an attribute from the edges in a shortest path. For example:

library(igraph)
gdf <- data.frame(from = c(1,2,3,5), to = c(2,3,4,3), id = c("A","B","C","D"))
g <- graph_from_data_frame(gdf, directed = TRUE)
idx <- shortest_paths(g, 1, 3, output = "epath")$epath[[1]]
igraph::edge_attr(g, "id", idx)

Which returns:

[1] "A" "B"

If I want to get the labels for the shortest path to the end of the network:

idx <- shortest_paths(g, 1, 4, output = "epath")$epath[[1]]
igraph::edge_attr(g, "id", idx)

I get:

Warning message:
In shortest_paths(g1, 1, 4, output = "epath") :
  At structural_properties.c:740 :Couldn't reach some vertices

And idx is:

+ 0/4 edges from b8aaa81 (vertex names):

For this last one I would have expected to get [1] "A" "B" "C"

How do I get the labels for a shortest path that includes the final vertex of the graph?

标签： rigraph

Answer 1

问题是您引用顶点的方式。请注意，顶点 ID 不与顶点标签对齐。

vertex_attr(g)
$name
[1] "1" "2" "3" "5" "4"

当你写shortest_paths(g, 1, 4, output = "epath")$epath[[1]]

它将采用标签为“1”和“5”的第一个和第四个节点。这是失败的，因为没有从“1”到“5”的路径。我相信你的意思是

idx <- shortest_paths(g, "1", "4", output = "epath")$epath[[1]]
igraph::edge_attr(g, "id", idx)
[1] "A" "B" "C"

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