What happens, and what did you expect instead?

Several function that return vertex or edge sequences ignore the value of the option return.vs.es and return igraph.vs or igraph.es objects even if return.vs.es = FALSE. A few examples are demonstrated below.

I used two regex pattern to search of code that converts numeric indices to igraph.vs or igraph.es objects, respectively:

• "create_(v|e)s" finds uses of create_vs() and create_es() (also when they are used inside lapply() etc.).
• "(E|V)\([^)]\)\[" finds constructs of the form E(graph)[i + 1]

Most uses of those patterns are already combined with a check for the value of return.vs.es, but there are a few exceptions:

• head_of()
• tail_of()
• as_adj_edge_list()
• graph.get.isomorphisms.vf2()
• graph.get.subisomorphisms.vf2()
• graph.subisomorphic.lad()

I would expect that these functions return (lists of) numeric indices if return.vs.es = FALSE.

To reproduce

library(igraph)
#> 
#> Attaching package: 'igraph'
#> The following objects are masked from 'package:stats':
#> 
#>     decompose, spectrum
#> The following object is masked from 'package:base':
#> 
#>     union

g <- make_tree(6, children = 2)
V(g)$name <- paste0("V", 1:6)

# make sure the option is set to the default
igraph_options(return.vs.es = TRUE)

# in this case, all results are as expected
head_of(g, E(g)[c(1, 4)])
#> + 2/6 vertices, named, from d6aad98:
#> [1] V2 V5
tail_of(g, E(g)[c(1, 4)])
#> + 2/6 vertices, named, from d6aad98:
#> [1] V1 V2
as_adj_edge_list(g)[1]
#> $V1
#> + 2/5 edges from d6aad98 (vertex names):
#> [1] V1->V2 V1->V3
graph.get.isomorphisms.vf2(g, g)[1]
#> [[1]]
#> + 6/6 vertices, named, from d6aad98:
#> [1] V1 V2 V3 V4 V5 V6
graph.get.subisomorphisms.vf2(g, g)[1]
#> [[1]]
#> + 6/6 vertices, named, from d6aad98:
#> [1] V1 V2 V3 V4 V5 V6
graph.subisomorphic.lad(g, g, all.maps = TRUE)$maps[1]
#> [[1]]
#> + 6/6 vertices, named, from d6aad98:
#> [1] V1 V2 V3 V4 V5 V6

# changing the option does not affect the results
igraph_options(return.vs.es = FALSE)

head_of(g, E(g)[c(1, 4)])
#> + 2/6 vertices, named, from d6aad98:
#> [1] V2 V5
tail_of(g, E(g)[c(1, 4)])
#> + 2/6 vertices, named, from d6aad98:
#> [1] V1 V2
as_adj_edge_list(g)[1]
#> $V1
#> + 2/5 edges from d6aad98 (vertex names):
#> [1] V1->V2 V1->V3
graph.get.isomorphisms.vf2(g, g)[1]
#> [[1]]
#> + 6/6 vertices, named, from d6aad98:
#> [1] V1 V2 V3 V4 V5 V6
graph.get.subisomorphisms.vf2(g, g)[1]
#> [[1]]
#> + 6/6 vertices, named, from d6aad98:
#> [1] V1 V2 V3 V4 V5 V6
graph.subisomorphic.lad(g, g, all.maps = TRUE)$maps[1]
#> [[1]]
#> + 6/6 vertices, named, from d6aad98:
#> [1] V1 V2 V3 V4 V5 V6

^{Created on 2024-12-02 with reprex v2.1.1}

System information

R version 4.4.2 (2024-10-31)
Platform: x86_64-pc-linux-gnu
Running under: Ubuntu 24.04.1 LTS

Matrix products: default
BLAS: /usr/lib/x86_64-linux-gnu/blas/libblas.so.3.12.0
LAPACK: /usr/lib/x86_64-linux-gnu/lapack/liblapack.so.3.12.0

locale:
[1] LC_CTYPE=en_GB.UTF-8 LC_NUMERIC=C LC_TIME=de_CH.UTF-8
[4] LC_COLLATE=en_GB.UTF-8 LC_MONETARY=de_CH.UTF-8 LC_MESSAGES=en_GB.UTF-8
[7] LC_PAPER=de_CH.UTF-8 LC_NAME=C LC_ADDRESS=C
[10] LC_TELEPHONE=C LC_MEASUREMENT=de_CH.UTF-8 LC_IDENTIFICATION=C

time zone: Europe/Zurich
tzcode source: system (glibc)

attached base packages:
[1] stats graphics grDevices utils datasets methods base

other attached packages:
[1] igraph_2.1.2

loaded via a namespace (and not attached):
[1] compiler_4.4.2 magrittr_2.0.3 cli_3.6.3 tools_4.4.2 rstudioapi_0.17.1
[6] lifecycle_1.0.4 pkgconfig_2.0.3 rlang_1.1.4