Obtain an edges vector in a form suitable for make_graph()

[szhorvat]

What is the feature or improvement you would like to see?

There should be a function that retrieves an edge list in a form suitable for make_graph(), as well as suitable for the C function igraph_create().

as_edgelist() returns a matrix, not a vector, and converting to a vector gives the vertex IDs in the wrong order:

> g<-make_graph(c(1,2, 2,3, 3,4, 4,5))
> as.vector(as_edgelist(g, names=F))
[1] 1 2 3 4 2 3 4 5

Instead one must transpose the matrix first, which is inefficient:

> as.vector(t(as_edgelist(g, names=F)))
[1] 1 2 2 3 3 4 4 5

A more efficient way is

> .Call(igraph:::R_igraph_get_edgelist, g, F)
[1] 0 1 1 2 2 3 3 4

but this uses an internal function.

Use cases for the feature

For libraries that need to convert igraph graphs efficiently, such as leidenbase or leidenalg.

References

• Test failures with the development version of igraph cole-trapnell-lab/leidenbase#39
• Test failures with the development version of igraph kharchenkolab/leidenAlg#13

[schochastics]

This is as_edgelist

as_edgelist <- function(graph, names = TRUE) {
  ensure_igraph(graph)
  on.exit(.Call(R_igraph_finalizer))
  res <- matrix(.Call(R_igraph_get_edgelist, graph, TRUE), ncol = 2)
  res <- res + 1
  if (names && "name" %in% vertex_attr_names(graph)) {
    res <- matrix(V(graph)$name[res], ncol = 2)
  }

  res
}

Simple solution would be to a add a bool (just arg here) that defaults to TRUE?

as_edgelist <- function(graph, names = TRUE, arg = TRUE) {
  ensure_igraph(graph)
  on.exit(.Call(R_igraph_finalizer))
  res <- matrix(.Call(R_igraph_get_edgelist, graph, TRUE), ncol = 2)
  res <- res + 1
  if(arg){
	res <- matrix(.Call(R_igraph_get_edgelist, graph, TRUE), ncol = 2)
  	if (names && "name" %in% vertex_attr_names(graph)) {
      res <- matrix(V(graph)$name[res], ncol = 2)
  	}
  } else{
    res <- .Call(R_igraph_get_edgelist, graph, FALSE)
    if (names && "name" %in% vertex_attr_names(graph)) {
      res <- V(graph)$name[res]
  	}
  }

  res
}