I must be drowning in a cup of tea, but at the same time I have been banging my head against the wall for hours. Please have a look at the reprex below where I generate an igraph graph object g and I collect the edges stemming from some vertices into an object called dd. My goal is to convert to convert dd into a dataframe or a tibble but the most obvious methods (as_data_frame and as_tibble) fail completely. Can anyone help me out?
Many thanks
library(tidyverse)
library(igraph)
#>
#> Attaching package: 'igraph'
#> The following objects are masked from 'package:dplyr':
#>
#> as_data_frame, groups, union
#> The following objects are masked from 'package:purrr':
#>
#> compose, simplify
#> The following object is masked from 'package:tidyr':
#>
#> crossing
#> The following object is masked from 'package:tibble':
#>
#> as_data_frame
#> The following objects are masked from 'package:stats':
#>
#> decompose, spectrum
#> The following object is masked from 'package:base':
#>
#> union
g <- make_ring(10)
dd <- E(g)[from(1:4)]
## this does not work
dd%>%as_tibble
#> Error in as.data.frame.default(value, stringsAsFactors = FALSE): cannot coerce class '"igraph.es"' to a data.frame
## neither this
dd%>%as_data_frame
#> Error in as_data_frame(.): Not a graph object
## so what to do?
Created on 2021-07-31 by the reprex package (v2.0.0)
You have extracted edge identifiers from the graph-object. Now you need to understand how the graphs and edges are represented. So use the plot and str commands to examine the details of the g and dd objects:
> g <- make_ring(10)
> plot(g)
> dd <- E(g)[from(1:4)]
> dd
+ 5/10 edges from 96250aa:
[1] 1-- 2 2-- 3 3-- 4 4-- 5 1--10
> str(dd)
'igraph.es' int [1:5] 1 2 3 4 10 # so it's really just a numeric vector
- attr(*, "env")=<weakref>
- attr(*, "graph")= chr "96250aab-d2ab-43aa-882d-6ef695323b0a"
> help(pack=igraph)
> as_edgelist(g)
[,1] [,2] # whereas the original graph (list) converts to two columns
[1,] 1 2
[2,] 2 3
[3,] 3 4
[4,] 4 5
[5,] 5 6
[6,] 6 7
[7,] 7 8
[8,] 8 9
[9,] 9 10
[10,] 1 10
> as_edgelist(dd)
Error in as_edgelist(dd) : Not a graph object
After looking at the results I opened the igraph help index and noted a function named as_ids that was described as: "Convert a vertex or edge sequence to an ordinary vector"
> as_ids(dd)
[1] 1 2 3 4 10
> str(g)
List of 10
$ :List of 1
..$ : 'igraph.vs' int [1:2] 2 10
.. ..- attr(*, "env")=<weakref>
.. ..- attr(*, "graph")= chr "96250aab-d2ab-43aa-882d-6ef695323b0a"
$ :List of 1
..$ : 'igraph.vs' int [1:2] 1 3
.. ..- attr(*, "env")=<weakref>
.. ..- attr(*, "graph")= chr "96250aab-d2ab-43aa-882d-6ef695323b0a"
$ :List of 1
..$ : 'igraph.vs' int [1:2] 2 4
.. ..- attr(*, "env")=<weakref>
.. ..- attr(*, "graph")= chr "96250aab-d2ab-43aa-882d-6ef695323b0a"
$ :List of 1
..$ : 'igraph.vs' int [1:2] 3 5
.. ..- attr(*, "env")=<weakref>
.. ..- attr(*, "graph")= chr "96250aab-d2ab-43aa-882d-6ef695323b0a"
$ :List of 1
..$ : 'igraph.vs' int [1:2] 4 6
.. ..- attr(*, "env")=<weakref>
.. ..- attr(*, "graph")= chr "96250aab-d2ab-43aa-882d-6ef695323b0a"
$ :List of 1
..$ : 'igraph.vs' int [1:2] 5 7
.. ..- attr(*, "env")=<weakref>
.. ..- attr(*, "graph")= chr "96250aab-d2ab-43aa-882d-6ef695323b0a"
$ :List of 1
..$ : 'igraph.vs' int [1:2] 6 8
.. ..- attr(*, "env")=<weakref>
.. ..- attr(*, "graph")= chr "96250aab-d2ab-43aa-882d-6ef695323b0a"
$ :List of 1
..$ : 'igraph.vs' int [1:2] 7 9
.. ..- attr(*, "env")=<weakref>
.. ..- attr(*, "graph")= chr "96250aab-d2ab-43aa-882d-6ef695323b0a"
$ :List of 1
..$ : 'igraph.vs' int [1:2] 8 10
.. ..- attr(*, "env")=<weakref>
.. ..- attr(*, "graph")= chr "96250aab-d2ab-43aa-882d-6ef695323b0a"
$ :List of 1
..$ : 'igraph.vs' int [1:2] 1 9
.. ..- attr(*, "env")=<weakref>
.. ..- attr(*, "graph")= chr "96250aab-d2ab-43aa-882d-6ef695323b0a"
- attr(*, "class")= chr "igraph"
So if you wanted a data.frame to hold the edge identifiers, you could get a dataframe with a single column, by just doing:
dd_df <- data.frame( ids <- as_ids(dd) )
But if you wanted a two column result that use as_data_frame method for graph object and then select the first 4 rows of the graph:
> as_data_frame(g)[1:4, ]
from to
1 1 2
2 2 3
3 3 4
4 4 5