Why does Ruby allow me to push an array on itself?

Question

This code is valid in Ruby

a = [5,10,15]
[5,10,15]
a.push a
[5,10,15,[...]]

Resulting in the fourth array slot pointing to the array itself, (seemingly) infinitely. Why does Ruby allow this and does the functionality offer any practical applications?

The simplest example I can think of is a garbage collector - you keep a list of references to every object in memory (including that list!) — roippi, Apr 28 '14 at 15:35

score 3 · Accepted Answer · edited May 23 '17 at 12:32

3

Since in Ruby everything is an object, variables just point to the objects (more strictly speaking, memory locations). An array is a collection of such a pointers, which means it can store a pointer to itself. It is not an extra feature added in Ruby, it would be actually an extra feature not to allow it.

As for application, check out "What are recursive arrays good for?" (directed graph representation).

Note however, that such an array is not infinite:

a = []
a << a
a.length = 1

edited May 23 '17 at 12:32

Community

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answered Apr 28 '14 at 15:29

BroiSatse

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Are there any useful applications? – Robert Harvey Apr 28 '14 at 15:31

score 0 · Answer 2 · edited Apr 28 '14 at 18:59

0

Since Ruby is a dynamic language, an array is, in essence, a collection of "any object" so you can push anything you want into it, including other arrays, including (in this case) a reference to itself. It's like an ArrayList<Object> in Java, which can do the same thing (you can add it to itself, but why?)

It might be sometimes useful to have recursive structures, though nothing comes to mind.

edited Apr 28 '14 at 18:59

the Tin Man

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answered Apr 28 '14 at 15:28

rogerdpack

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Why does Ruby allow me to push an array on itself?

2 Answers2