How to use a recursive array

Question

I have array, named a and define it with [1, 2, 3].

Next, I pushed it to itself:

a = [1, 2, 3]
a << a

and the result I get is:

#=> [1, 2, 3, [...]]

When I want to get the last element of array using a.last I get:

a.last
#=> [1, 2, 3, [...]]
#even 
a.last.last.last
#=> [1, 2, 3, [...]]

What is going on, when we would push array to itself?

Yes, I understand that this should create a recursive array, but what can we do with it?

"what we can do with it" – make coffee if you type enough .last and make the CPU produce enough heat…? I'm not sure what kind of answer you're expecting here… — deceze
– deceze ♦, Commented Oct 31, 2016 at 13:40
@deceze I mean, maybe someone know how to use recursive array in real work examples — Oleksandr Holubenko
– Oleksandr Holubenko, Commented Oct 31, 2016 at 13:43
I'm sure there are real world use cases for this, and you will know them when you encounter them. Coming up with random examples is not really a productive question for SO. — deceze
– deceze ♦, Commented Oct 31, 2016 at 13:44
They're pretty useful in Mathematics, as couter-examples. Your recursive array is basicaly infinitely long. If you make one step towards infinity, you basically didn't move at all ;) — Eric Duminil
– Eric Duminil, Commented Oct 31, 2016 at 13:45

sepp2k · Accepted Answer · 2016-10-31 13:42:37Z

2

In Ruby variables, array elements etc. are object references. So when you do a = [1, 2, 3], there will be an array somewhere in memory and the a variable is a reference to that memory. Now when you do a << a, a[4] will also be a reference to that object. So in effect a now contains a reference to itself.

answered Oct 31, 2016 at 13:42

sepp2k

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Ursus · Accepted Answer · 2016-10-31 13:41:34Z

1

a = [1, 2, 3]
a << a.dup
a.last
=> [1, 2, 3]  
a.last.last
=> 3

Maybe this is what you wanted. This just insert an array [1, 2, 3] as the last item of the a array. In the way you did you put a reference at the end of the a array and this becomes recursive.

answered Oct 31, 2016 at 13:41

Ursus

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