Subset an igraph object for the name of the edge

Question

Consider a data frame that correlated each variable from with each other:

iris_cor <- structure(list(x = c("Sepal.Length", "Sepal.Length", "Sepal.Length", 
"Sepal.Length", "Sepal.Width", "Sepal.Width", "Sepal.Width", 
"Sepal.Width", "Petal.Length", "Petal.Length", "Petal.Length", 
"Petal.Length", "Petal.Width", "Petal.Width", "Petal.Width", 
"Petal.Width"), y = c("Sepal.Length", "Sepal.Width", "Petal.Length", 
"Petal.Width", "Sepal.Length", "Sepal.Width", "Petal.Length", 
"Petal.Width", "Sepal.Length", "Sepal.Width", "Petal.Length", 
"Petal.Width", "Sepal.Length", "Sepal.Width", "Petal.Length", 
"Petal.Width"), r = c(NA, -0.117569784133002, 0.871753775886583, 
0.817941126271576, -0.117569784133002, NA, -0.42844010433054, 
-0.366125932536439, 0.871753775886583, -0.42844010433054, NA, 
0.962865431402796, 0.817941126271576, -0.366125932536439, 0.962865431402796, 
NA)), row.names = c(NA, -16L), class = c("tbl_df", "tbl", "data.frame"
), .Names = c("x", "y", "r"))

If I wanted to make a network plot of this I would do the following:

library(igraph)
iris_igraph <- graph_from_data_frame(iris_cor)

Now if I were only interested in the correlations that involved with Sepal.Length I am unclear on how to efficiently extract that information. Trying both of these approaches is not successful. The first because of the error and the second because I only get the first instance of Sepal.Length not all of them:

> subgraph.edges(iris_igraph, E(iris_igraph)[name == "Sepal.Length"])
Error in eval(x$expr, data, x$env) : object 'name' not found
> 
> subgraph.edges(iris_igraph, V(iris_igraph)[name == "Sepal.Length"])
IGRAPH dc7408b DN-- 1 1 -- 
+ attr: name (v/c), r (e/n)
+ edge from dc7408b (vertex names):
[1] Sepal.Length->Sepal.Length

This results in the desired output:

subgraph.edges(iris_igraph, E(iris_igraph)[1:4])

However a numbered index is not practical when the desired subsets of the igraph object aren't in order or there are many nodes/edges.

Can anyone recommend a way to subset an igraph object for the name of the edge?

Answer 1

According to indexing edge sequences , you can use the special function inc for edges subsetting, which:

takes a vertex sequence, and selects all edges that have at least one incident vertex in the vertex sequence.

E(iris_igraph)[inc('Sepal.Length')]
#+ 7/16 edges (vertex names):
#[1] Sepal.Length->Sepal.Length Sepal.Length->Sepal.Width  Sepal.Length->Petal.Length
#[4] Sepal.Length->Petal.Width  Sepal.Width ->Sepal.Length Petal.Length->Sepal.Length
#[7] Petal.Width ->Sepal.Length

Select edges from Sepal.Length :

E(iris_igraph)[from('Sepal.Length')]
#+ 4/16 edges (vertex names):
#[1] Sepal.Length->Sepal.Length Sepal.Length->Sepal.Width  Sepal.Length->Petal.Length
#[4] Sepal.Length->Petal.Width 

Select edges to Sepal.Length :

E(iris_igraph)[to('Sepal.Length')]
#+ 4/16 edges (vertex names):
#[1] Sepal.Length->Sepal.Length Sepal.Width ->Sepal.Length Petal.Length->Sepal.Length
#[4] Petal.Width ->Sepal.Length