Overwriting attributes converts the new value to the class of the old value

[arcresu]

When setting an attribute that already exists (using V()<- or set_vertex_attr() and presumably also with edge attributes), igraph (1.2.6) will convert the new value to match the class of any existing value. For example:

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

# does some processing and stores the result in a new integer attribute 'kind'
set_kind <- function(gr) {
  # gr <- delete_vertex_attr(gr, "kind")  # workaround
  set_vertex_attr(gr, "kind", value = vcount(gr))
}

# (1) processing a clean graph works as expected
g <- make_full_graph(n=1)
g <- set_kind(g)
class(V(g)$kind)
#> [1] "numeric"

# (2) this graph happens to have an existing attribute 'kind' with a different class;
#     the result now has the wrong class
g <- make_full_graph(n=1) %>% set_vertex_attr("kind", value = "big")
g <- set_kind(g)
class(V(g)$kind)
#> [1] "character"

^{Created on 2021-06-10 by the reprex package (v2.0.0)}

Basically I would expect the parts marked (1) and (2) to do the same thing, but the result actually depends on whatever value the attribute had before. This makes V() <- inconsistent with the normal R semantics of the <- operator which allow you to replace a variable with a value of a different class just fine.

To make the processing code robust against arbitrary input graphs, it's necessary to explicitly clear every attribute before setting it, but this becomes cumbersome.

[ntamas]

FWIW it looks like character cannot be converted to integer simply by doing an assignment, but it works the other way round:

# (1) converting a character attribute to integer doesn't work
> g <- make_full_graph(n=1) %>% set_vertex_attr("kind", value = "big")
> class(V(g)$kind)
#> [1] "character"
> V(g)$kind <- vcount(g)
> class(V(g)$kind)
#> [1] "character"
> V(g)$kind
#> [1] "1"

# (2) but converting an integer to a character does
> g <- make_full_graph(n=1) %>% set_vertex_attr("kind", value = vcount(g))
> class(V(g)$kind)
#> [1] "integer"
> V(g)$kind <- c("big")
> class(V(g)$kind)
#> [1] "character"

[krlmlr]

I'm about to enter a deep deep rabbit hole here.

To fix this in an idiomatic way, we need to be able to distinguish between:

• updating the attributes for all vertices (for which we would allow changing the type), and
• updating the attributes for only a subset of the vertices (for which we would be stricter and only allow casts to more generic types, e.g., from integer to character).

To support this in a strict way, V(g) would need to return an object that indicates that all vertices are addressed. Currently, V(g) isn't much different from V(g)[1:3] :

suppressMessages(library(igraph))
g <- make_ring(10)
dput(V(g))
#> structure(1:10, class = "igraph.vs", env = <weak reference>, graph = "79e455be-99fd-414c-a447-fcd86cdbacee")
dput(V(g)[1:3])
#> structure(1:3, env = <weak reference>, graph = "79e455be-99fd-414c-a447-fcd86cdbacee", class = "igraph.vs")
dput(V(g)[2:5])
#> structure(2:5, env = <weak reference>, graph = "79e455be-99fd-414c-a447-fcd86cdbacee", class = "igraph.vs")

^{Created on 2022-12-14 with reprex v2.0.2}

Ideally, V(g) would return an object that allows us to make this distinction. This could be as simple as an extra attribute (for maximum compatibility).

Should we go all in here?

[ntamas]

I guess yes; V(g) could return a structure where is_all = TRUE and any subsetting of V(g) could set it to is_all = FALSE; would that work? I'm not sure about the performance implications but it does not seem too severe.

[krlmlr]

Preparatory refactorings are done now, one final step still to do. We want to decide first if we can afford stricter recycling rules, a revdepcheck seems necessary.

[krlmlr]

Might want to revert the most recent PR #608 too, #607 is gone already.

[krlmlr]

Do as.igraph.vs() and as.igraph.es() need to carry over the new is_all attribute?

library(igraph, warn.conflicts = FALSE)

g <- make_full_graph(n = 1)

dput(V(g))
#> structure(1L, class = "igraph.vs", is_all = TRUE, env = <weak reference>, graph = "eb0db5f3-100d-462b-b8e2-bea733367953")
dput(igraph:::as.igraph.vs(g, V(g)))
#> 1

^{Created on 2023-01-15 with reprex v2.0.2}

[krlmlr]

Is this intended?

library(igraph, warn.conflicts = FALSE)

g <- make_full_graph(n = 10)

V(g)$foo <- 1
V(g)$foo
#>  [1] 1 1 1 1 1 1 1 1 1 1

vertex.attributes(g, 2:4) <- list(foo = 2:4)

V(g)$foo
#>  [1] NA  2  3  4 NA NA NA NA NA NA

^{Created on 2023-01-15 with reprex v2.0.2}