Somebody tell me what's going on here:
a = [0,1,2]
a.each {|x| a[x] = a}
The result is [[...], [...], [...]]. And if I evaluate a[0] I get [[...], [...], [...]]. And if I evaluate a[0][0] I get [[...], [...], [...]] ad infinitum.
Have I created an array of infinite dimensionality? How/Why should this possibly work?
Basically you've modified every element in a to reference the list itself. The list is recursively referencing itself:
a[0] # => a
a[0][0] # => a[0], which is a
a[0][0][0] # => a[0][0], which is a[0], which is a
...
(# => is a Rubyism for "this line evaluates to")
Depending on how you look at it it is not infinite. It's more or less just like a piece of paper with the words "please turn over" written on both sides.
The reason that Ruby prints [...] is that it is clever enough to discover that the list is recursive, and avoids going into an infinite loop.
By the way, your usage of each is a bit non-idiomatic. each returns the list, and you usually don't assign this return value to a variable (since you already have a variable referencing it, a in this case). In other words, your code assigns [0,1,2] to a, then loops over a (setting each element to a), then assigns a to a.
I think it's a self-referential data structure. The a[x]=a puts a's pointer in a[x].
a = []; a[0] = a; a[1] = a; a[2] = awill have exactly the same result.blk = proc { |i| Array.new(10, &blk) }; a = Array.new(10, &blk). This causes a SystemStackError though as Ruby can't handle infinite recursion.