When i write the following code:
x= [1,2,3]
x << x
puts x
puts x[3]
puts x[3][3][3][3][3][3][3][3][3][3]
I get this output:
[1, 2, 3, [...]]
[1, 2, 3, [...]]
[1, 2, 3, [...]]
Shouldn't I get only [1,2,3,[1,2,3]] and what would be the explanation?
There's nothing strange about this. The fourth element of the array is the array itself, so when you ask for the fourth element, you get the array, and when you ask for the fourth element of the fourth element, you get the array, and when you ask for the fourth element of the fourth element of the fourth element of the fourth element ... you get the array.
Simple as that.
The only slightly unusual thing is that Array#to_s detects such recursion and instead of going into an infinite loop, returns an ellipsis.
Array#<=>, #eql? and #hash also deal with recursion, same for Hash and Struct... Not sure why you consider it "unusual"! blog.marc-andre.ca/2009/05/lost-in-recursion.html
to_s implementations, but I never considered special casing for that. If someone asks for a string representation of an infinite data structure, they'll get an infinite loop. Period. I mean, map for an infinite Enumerator returns an infinitely large array, and nobody thinks twice about that, isn't that somewhat the same?
When you write x << x you are appending to x with a reference to itself, creating a recursive/infinite array.
Compare this with x << x.dup which appends a copy of x onto x. This is equal to [1,2,3,[1,2,3]]
x+=x ? I mean to create an other kind of recursive array.
x+=x is equivalent to x = x + x, which means create a new array by joining the elements from x with the elements from x and then make x refer to this newly created array so you end up with [1, 2, 3, 1, 2, 3]. << pushes the given object onto the end of an array (no new array is created.) See the docs for + and docs for <<.