Question

I am trying to return a subset of a multi-dimensional array, trying to keep the exact structure of dimensions, but.. something strange is happening… take a look please:

space = [  [ [1],[2],[3] ],  [ [4],[5],[6] ],  [ [70],[8],[9] ]  ]

space_subset = space[(1..2)].collect { |y| y[1] }

=> [[5], [8]] 

Let’s break it down:

space[(1..2)]

=> [  [ [4], [5], [6] ], [ [70], [8], [9] ]  ]

so now I can be sure what I am calling .collect on

in fact:

[  [ [4], [5], [6] ], [ [70], [8], [9] ]  ].collect { |y| y[1] }

=> [[5], [8]]

Then… (for the real question)…

If now space_subset is [[5], [8]]

and I try to modify it like this:

space_subset[1].delete (8)

and as expected I get: => [[5], []]

why does this at the same time modifies the original “space” array from which I extracted the subset array ?

If now I do:

space

=> [[[1], [2], [3]], [[4], [5], [6]], [[70], [], [9]]]

“8” is missing, the same value I deleted from the space_subset

I am looking at ruby Array api docs and from what I am reading my code should work without surprises… but.. still…..

Can you help me figure what I’m doing wrong, or misunderstanding here ?

Thanks to everyone who takes the time to answer

Answer 1

This is a difference between reference and value. In your code, you create a reference to the inner arrays, but you are referencing the same values in both places. You can confirm this by calling Object#object_id on both arrays (as if changing the value via one reference and seeing be modified from the other reference isn’t confirmation enough!).

space = [  [ [1],[2],[3] ],  [ [4],[5],[6] ],  [ [70],[8],[9] ]  ]
=> [[[1], [2], [3]], [[4], [5], [6]], [[70], [8], [9]]] 
space[2][1].object_id
=> 70329700053380 
space_subset = space[(1..2)].collect { |y| y[1] }
=> [[5], [8]] 
space_subset[1].object_id
=> 70329700053380

Unfortunately, Array#dup and Array#clone only make “shallow” copies of objects, so you have to use a bit of a workaround to get a copy of space to work with. One easy trick to get a deep copy is:

Marshal.load(Marshal.dump(space))

You can also write a recursive function to take space and manually copy it into a new array.

And just to prove it:

space = [  [ [1],[2],[3] ],  [ [4],[5],[6] ],  [ [70],[8],[9] ]  ]
=> [[[1], [2], [3]], [[4], [5], [6]], [[70], [8], [9]]]
space[2][1].object_id
=> 70329700053380
space_subset = Marshal.load(Marshal.dump(space))
=> [[[1], [2], [3]], [[4], [5], [6]], [[70], [8], [9]]]
space_subset = space_subset[(1..2)].collect { |y| y[1] }
=> [[5], [8]]
space_subset[1].object_id
=> 70329695297500
space_subset[1].delete(8)
=> 8
space
=> [[[1], [2], [3]], [[4], [5], [6]], [[70], [8], [9]]]
space_subset
=> [[5], []]

Hope that helps!