added graph_from_edges function

[Sequenzer]

Added a function that creates a graph from a set of edges.

[lgoettgens]

How can I create a graph with isolated vertices using this function? Maybe one can use something like permutation constructors from cycles where there are fixpoints in the permutation?

[Sequenzer]

How can I create a graph with isolated vertices using this function? Maybe one can use something like permutation constructors from cycles where there are fixpoints in the permutation?

I have added an optional argument called additional_vertices, which you can use to create isolated vertices.

[lkastner]

I think it makes more sense to just allow an optional n_vertices:

Suggested change

	function graph_from_edges(::Type{T}, edges::Vector{Edge}, additional_vertices::Vector{Int}=Int[]) where {T <: Union{Directed, Undirected}}
	function graph_from_edges(::Type{T}, edges::Vector{Edge}; n_vertices::Int=-1) where {T <: Union{Directed, Undirected}}

and then later something like

n = maximum(verts)
if n_vertices>-1
  @req n_vertices >= n "Edges require more vertices than allowed."
  n = maximum(n, n_vertices)
end

My reasons are the following:

• If you call graph_from_edges([[1,3]]) it will have an isolated vertex at 2 anyway. (You do this vertex removal later, but why would you want that? It seems intransparent to the user, maybe make it a separate thing. polymake has squeeze for that.)
• Graphs always have vertices 1...n.
• You never check whether add_vertex! returns false. You should probably also check this for add_edge anyway.

Maybe also set a default for T?

[Sequenzer]

I disagree, the change to an optional argument n_vertices once again puts the effort on the user to calculate the total amount his edges plus desired isolated vertices require. This completely defeats the purpose of a highly requested, easy to use function to create a graph. I know it is a trivial thing to do, but still work to be done.

Similarly, it feels much more counterintuitive that graph_from_edges([[1,3]]) will give you an unwanted isolated vertex, rather than just a graph with one edge and two vertices. I will think about a way to make vertex removal transparent for the user.

I will check out squeeze from Polymake and add a default type for T. Thanks a lot :)

[codecov]

Codecov Report

Merging #3300 (156bbb8) into master (2e37496) will increase coverage by 0.00%.
The diff coverage is 90.90%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3300   +/-   ##
=======================================
  Coverage   81.71%   81.71%           
=======================================
  Files         556      556           
  Lines       74130    74141   +11     
=======================================
+ Hits        60573    60583   +10     
- Misses      13557    13558    +1     

Files	Coverage Δ
src/Combinatorics/Graphs/functions.jl	93.33% <90.90%> (-0.16%)	⬇️