[WIP] Adding vertex names when a named and unnamed graph are combined?

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original issue description:

See discussion in #1908 and #1932 (also related: #60)

What should be the strategy when a named and unnamed graph are combined by whatever means
(e.g. disjoint_union(), a graph product, "+",...)

I see two options:

1. throw an error (consistent with throwing an error when duplicated names are found)
2. create generic vertex names. An example implementation is at the bottom.

Currently, names are set to NA, which is in my opinion not desirable.

library(igraph)
library(purrr)

consolidate_names <- function(graphs, prefix = "V") {
  have_names <- map_lgl(graphs, function(g) "name" %in% vertex_attr_names(g))
  if (all(have_names)) {
    all_names <- list_c(map(graphs, function(g) V(g)$name))

    if (any(duplicated(all_names))) {
      cli::cli_abort("Duplicated vertex names found across graphs.")
    }
    return(graphs)
  }

  existing_names <- list_c(map(graphs, function(g) V(g)$name))

  # Create a counter for generating new names (check if named graph already has generic names)
  name_counter <- if (any(grepl(paste0(prefix, "[0-9]+"), existing_names))) {
    max(as.integer(gsub(paste0(prefix, "([0-9]+)"), "\\1", existing_names)), na.rm = TRUE) + 1
  } else {
    1
  }

  for (i in seq_along(graphs)) {
    if (!have_names[i]) {
      n <- vcount(graphs[[i]])
      num_id <- seq(name_counter, length.out = n)
      V(graphs[[i]])$name <- paste0(prefix, num_id)
      name_counter <- name_counter + n + 1
    }
  }

  return(graphs)
}

g1 <- g2 <- make_ring(4)
V(g1)$name <- c("A", "B", "C", "D")
graphs <- list(g1, g2)
consolidate_names(graphs)
#> [[1]]
#> IGRAPH 467efa4 UN-- 4 4 -- Ring graph
#> + attr: name (g/c), mutual (g/l), circular (g/l), name (v/c)
#> + edges from 467efa4 (vertex names):
#> [1] A--B B--C C--D A--D
#> 
#> [[2]]
#> IGRAPH 467efa4 UN-- 4 4 -- Ring graph
#> + attr: name (g/c), mutual (g/l), circular (g/l), name (v/c)
#> + edges from 467efa4 (vertex names):
#> [1] V1--V2 V2--V3 V3--V4 V1--V4

V(g1)$name <- c("V1", "V2", "V3", "V4")
graphs <- list(g1, g2)
consolidate_names(graphs)
#> [[1]]
#> IGRAPH 467efa4 UN-- 4 4 -- Ring graph
#> + attr: name (g/c), mutual (g/l), circular (g/l), name (v/c)
#> + edges from 467efa4 (vertex names):
#> [1] V1--V2 V2--V3 V3--V4 V1--V4
#> 
#> [[2]]
#> IGRAPH 467efa4 UN-- 4 4 -- Ring graph
#> + attr: name (g/c), mutual (g/l), circular (g/l), name (v/c)
#> + edges from 467efa4 (vertex names):
#> [1] V5--V6 V6--V7 V7--V8 V5--V8

^{Created on 2025-07-04 with reprex v2.1.1}

Fixes #1946.

---

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[krlmlr]

Nothing changed here, redo please.

[Copilot]

Nothing changed here, redo please.

I've implemented a solution for handling mixed named/unnamed graphs in disjoint_union(). The implementation adds logic to detect when some graphs have vertex names and others don't, then generates generic vertex names (V1, V2, etc.) for the unnamed graphs.

Key changes:

• Modified disjoint_union() in R/operators.R to consolidate names before graph combination
• Added comprehensive tests covering various scenarios
• Handles V-pattern name continuation properly

This addresses the issue described in #1946 by implementing option 2 (create generic vertex names) rather than throwing an error.

Commit: 19f69ed

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[krlmlr]

I think we want instructions about coding conventions for Copilot.