Errors while exporting and importing graphs

[ghost]

@aggelgian
I'm using the graph module to export & import graphs.
While exporting a graph, I sometimes get this error

** (ArgumentError) argument error
    (stdlib) :ets.select(262184, [{{:"$1", :_, :_, :_}, [], [:"$1"]}])
    (stdlib) digraph.erl:312: :digraph.edges/1
[…]

When I export the graph to a file and call :graph.from_file/('priv/graph.txt'), I get this error:

iex(20)⋅❯ :graph.from_file('priv/graph.txt')
** (MatchError) no match of right hand side value: {:error, {:fread, :integer}}
             :graph.from_file/3
    (stdlib) :digraph.del_edge/2

Right now my priority is the ability to import a graph. Could you tell me what I'm missing?

[aggelgian]

@Annwenn
Hello! How exactly do you export the graph?
Could you provide me with a minimal example to reproduce the error?

[ghost]

Yes sure, here's the code of the export_graph/2 function:

  def export_graph(graph, file \\ "priv/graph.txt") do
    n = :graph.num_of_vertices(graph)
    m = :graph.num_of_edges(graph)
    {:graph, t, _} = graph
    d = "d"
    first_line = ~s(#{n} #{m} #{t} #{d})
    data =
      :graph.edges_with_weights(graph) |>
        Enum.map_join("\n", fn(x) -> {{foo, bar}, w} = x ;
        "#{foo} #{bar} #{w}" end)
    content = """
    #{first_line}
    #{data}
    """
     IO.puts("Graph contains #{n} vertices, #{m} edges and is #{t}.")
     IO.inspect content
     File.write(file, content)
  end

Most of my functions are wrappers around yours and return the graph (which is stored as a state in the GenServer) so I can pipeline them with the |> (pipe) operator

iex(3)⋅❯ Uwotm8.add_user("foo")
{:graph, :directed, {:digraph, 290855, 294953, 299050, true}}
iex(4)⋅❯ Uwotm8.add_user("bar")
{:graph, :directed, {:digraph, 290855, 294953, 299050, true}}
iex(5)⋅❯ Uwotm8.add_user("baz")
{:graph, :directed, {:digraph, 290855, 294953, 299050, true}}
iex(6)⋅❯ Uwotm8.add_edge("foo", "bar", 20)
{:graph, :directed, {:digraph, 290855, 294953, 299050, true}}
iex(7)⋅❯ Uwotm8.add_edge("bar", "foo", 20)
{:graph, :directed, {:digraph, 290855, 294953, 299050, true}}
iex(8)⋅❯ Uwotm8.add_edge("bar", "baz", 80)
{:graph, :directed, {:digraph, 290855, 294953, 299050, true}}
iex(9)⋅❯ Uwotm8.print_graph
:ok
[{From, [{To, Weight}]}]
========================
iex(10)⋅❯ [{<<"bar">>,[{<<"foo">>,20},{<<"baz">>,80}]},
 {<<"baz">>,[]},
 {<<"foo">>,[{<<"bar">>,20}]}]
iex(10)⋅❯ Uwotm8.Worker.export_graph {:graph, :directed, {:digraph, 290855, 294953, 299050, true}}
Graph contains 3 vertices, 3 edges and is directed.
"3 3 directed d\nbar foo 20\nfoo bar 20\nbar baz 80\n"
:ok

Then the file looks like :

uniaika 17:53 ~/code/Elixir/POC/uwotm8 ❯❯ cat priv/graph.txt
3 3 directed d
bar foo 20
foo bar 20
bar baz 80

After a few corrections in the code, the error now looks like

iex(36)⋅❯ :graph.from_file('priv/graph.txt')
** (MatchError) no match of right hand side value: {:error, {:fread, :integer}}
             :graph.read_edge/2
             :graph.init_edges/6
             :graph.from_file/3
    (stdlib) :digraph.del_edge/2

[aggelgian]

@Annwenn

Hello!
Sorry for taking so long to address issue; I was really busy the last few weeks.

The problem is that graph:from_file/1 assumes that each vertex is an integer from 0 to N-1, where N is the number of vertices. Therefore, an exception occurs when trying to import vertices that are strings.

To facilitate importing / exporting graphs I just added two new MFAs:

• graph:export/3
  • The 1st argument is the graph.
  • The 2nd argument is the file.
  • The 3rd argument is a function that shows how to dump each vertex to a string.
• graph:import/2
  • The 1st argument is the file.
  • The 2nd argument is a function that shows how to parse a vertex from a string.

I've also added your use case an example in import_export.erl.

A possibly better example of importing a graph can be found in a_star_demo.

[ghost]

Woah, thank you!
Your library will come handy for my IRC bot. It has a Web of Trust module and I use the graph module to build relationships between users.
(here's the interesting part of the project.)

[aggelgian]

@Annwenn
I'm happy to hear that it will be useful for you. I'll check out your project ;)
If you need any other functionality, just tell me!

[ghost]

@aggelgian
Sure, thanks a lot!