What are recursive arrays good for?
arrays, recursion, ruby
Solution
A directed graph with undifferentiated edges could have each vertex represented simply as an array of the the vertices reachable from that vertex. If the graph had cycles, you would have a 'recursive array', especially if an edge could lead back to the same vertex.
For example, this graph: ...could be represented in code as:
nodes = { a:[], b:[], c:[], d:[] }
nodes[:a] << nodes[:a]
nodes[:a] << nodes[:b]
nodes[:b] << nodes[:a]
nodes[:b] << nodes[:c]
p nodes
#=> {:a=>[[[...], []], [...]], :b=>[[[...], [...]], []], :c=>[], :d=>[]}
Usually the representation of each vertex would be more 'robust' (e.g. a class instance with properties for the name and array of outgoing edges), but it's not impossible to imagine a case where you wanted a very lightweight representation of your data (for very large graphs) and so needed to use a minimal representation like this.
Problem
Ruby supports recursive arrays (that is, self-containing arrays): ``` a = [] # => [] a << a # => [[...]] a.first == a # => true ``` This is intrinsically cool, but what work can you do with it?