Indices of node in induced subgraphs

[AlexisSchOrange]

Hello,
I've an issue regarding the function induced_subgraph with metagraphs: in the metadata, the index of the new nodes is not changed correctly.

See the following example:

mg = MetaGraph(
    Graph(),
    Int,
    Dict,
    Dict,
    "mg"
)
add_vertex!(mg, 1, Dict(:name=>"Thierry Henry"))
add_vertex!(mg, 2, Dict(:name=>"Kyky Mbappe"))
add_vertex!(mg, 3, Dict(:name=>"Benjamin Pavard"))
add_edge!(mg, 1, 2, Dict(:type=>"Love"))
add_edge!(mg, 2, 3, Dict(:type=>"Adoration"))
add_edge!(mg, 3, 1, Dict(:type=>"Respect"))

submg = induced_subgraph(mg, [2, 3])[1]
for i in vertices(submg)
    print(submg[i])
end

Will print a KeyError: key 1 not found

Perhaps I'm not using this correctly ? Otherwise, I've been looking in the source code, the _copy_props! is not working correctly ?

Muchas gracias,

[gdalle]

Hi!
This comes from a confusion on using a MetaGraph as a dictionary. The syntax mg[i] retrieves the vertex associated to a given label (which can have arbitrary type), not to a given code, or index (which is always an integer). That is why the documentation recommends not using integers as labels as well, because it is very easy to mix up both concepts.
In the subgraph you create, the vertex labels remain [2, 3] but the vertex codes are renumbered to 1:2. More generally, labels are preserved through graph modifications, but codes are not.
To fix your program, here are two options:

julia> for l in labels(submg)
           println(l, " - ", submg[l])
       end
2 - Dict(:name => "Kyky Mbappe")
3 - Dict(:name => "Benjamin Pavard")

julia> for i in vertices(submg)
           println(i, " - ", submg[label_for(submg, i)])
       end
1 - Dict(:name => "Kyky Mbappe")
2 - Dict(:name => "Benjamin Pavard")

[AlexisSchOrange]

Thanks for the fast reply! It is more clear now. Perhaps I should create me a new package MetaGraphsButOnlyWithLabelThatAreSimpleIntegers.jl ... Have a good day