ruby lets you put an array inside itself
Submitted by musou in programming
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musou wrote
it's cool to know that python can do it too. i guess i should make a list of which languages can and can't do it.
it's super fun to think about, but i still can't come up with any way in which i could exploit it to do something fun or useful in order to make straightlaced programmers care about it
flabberghaster wrote
I think that it's basically a special case of how in those languages it's easy to assign a reference to things. Adding a list to itself is not really that different than assigning a reference to an object as a member of itself. For instance in python you can do:
class Node:
__init__(self, value, next=None):
self.value = value
self.next = next
def main():
head = Node("last")
tail = head
for i in range(10): head = Node(i, head)
tail.next = head
And this creates a cyclic linked list. You could just as easily assign a = Node(9); a.next = a and have the graph have only one vertex. It's the same principle with adding a list to itself: you're not actually adding the list to itself; rather you're putting a reference to the container, inside the container. Like writing down the location of a bank on a slip of paper, then putting that slip of paper inside a safe deposit box stored inside the same bank.
musou wrote
yeah! this is a good explanation of why it works. i'm mostly interested in figuring out ways to use recursive arrays that are practically useful, or at least interesting. but i still have nothing so far ._.
lainproliant wrote
What you're actually doing in Python when you put a list into itself is to put a reference to the list into itself.
L = []
L.append(L)
L[0] is L # True
Python and Ruby arrays are heterogenous, unlike in C++, where strong typing forces us to define a common type for objects contained in the same collection. There's no way to do this with the STL <vector>, but you could do this with an object system, for example Qt, where all "Q" objects derive from QObject:
QVector<QObject*> objectVector;
objectVector.append(&objectVector);
flabberghaster wrote
Turns out python can too. I was trying to think of how to get c++ to do it, but i could only think if how to do it using void pointers since to add something to a container that container would have to be a container of its own type which I can't think how to do off the top of my head.
Interesting problem.