Today’s Erlang Thursday starts to dig into the digraph module, as promised last week, and takes a look at digraph:add_vertex/1.

First we create a new directed graph, so we have something we can add vertices to.

Graph = digraph:new().
% {digraph,20498,24595,28692,true}

We then add some vertices to the graph by using digraph:add_vertex/1.

digraph:add_vertex(Graph).
% ['$v'|0]
digraph:add_vertex(Graph).
% ['$v'|1]
digraph:add_vertex(Graph).
% ['$v'|2]

As we don’t specify any information about the vertex we want to add, Erlang will create a new vertex for us of the format ['$v', I], with an empty list as the label where I is a non-negative integer.

We can also use digraph:add_vertex/2 to add a vertex if we wish to provide the vertex identifer, or provide vertex identifier and label in the case of digraph:add_vertex/3. As with digraph:add_vertex/1, digraph:add_vertex/2 uses the empty list as the label as well.

digraph:add_vertex(Graph, vertex1).
% vertex1
digraph:add_vertex(Graph, vertex2, "Vertex 2").
% vertex2

We have now added 5 vertices, and can check what vertices we have in the digraph() by using digraph:vertices/1.

digraph:vertices(Graph).
% [['$v'|2],['$v'|1],['$v'|0],vertex2,vertex1]

If we decide we want to try to add a vertex ourselves of the format ['$v' | I], we can run into trouble if you call digraph:add_vertex/1 after it.

digraph:add_vertex(Graph, ['$v' | 3]).
% ['$v'|3]
digraph:add_vertex(Graph).
% ['$v'|3]
digraph:vertices(Graph).
% [['$v'|2],['$v'|1],['$v'|0],['$v'|3],vertex2,vertex1]
digraph:add_vertex(Graph, ['$v' | 4]).
% ['$v'|4]
digraph:vertices(Graph).
% [['$v'|4],
%  ['$v'|2],
%  ['$v'|1],
%  ['$v'|0],
%  ['$v'|3],
%  vertex2,vertex1]

So we add a vertex by specifying the vertex() we want to add, and then add a new vertex and let Erlang take care of creating that vertex, and we wind up “losing” a vertex, as one essentially gets overridden when we look at the end state of the digraph().

–Proctor