Inscrutable Ruby: Vector Assignment Example

Question

Consider the following:

a=[0,1] #our starting value
a=[a,1] #=> [[0,1],1] as expected

I would anticipate the following to have the same result:

a=[0,1] #same starting place
a[0]=a  #should make a the same thing as it was above, right?
a       #=> [[...],1]   !!!

In the first example, the second assignment refers to the value of a before the assignment was made. In the second example, the second assignment performs a recursive assignment. This feels like different behavior to me. Is this behavior in fact consistent? If so can someone please explain why?

Answer 1

In the first example you are creating a new array with the value [[0,1], 1]. Then you are reassigning a to refer to this array.

In the second example you are not creating a new array, nor are you changing what a refers to. You are changing the existing array to contain a reference to itself. That's very different.

---

More details

The first example is roughly equivalent to this code:

a = [0, 1]  # Step 1
b = [a, 1]  # Step 2
a = b       # Step 3

In pictures it looks like this:

• Step 1 - create an array:

---
|a|
---
 |
 v
 [0, 1]

• Step 2 - create another array which includes a reference to the first:

    ---        ---
    |a|        |b| 
    ---        ---
     |          |
     |          v
     |          [ref, 1]
     |            |
     +------------+
     v 
     [0, 1]      

• Step 3 - change a to point to the array created in step 2:

    ---        ---
    |a|        |b| 
    ---        ---
     |          |
     +----------+
                v
                [ref, 1]
                  |
    +-------------+
    v 
    [0, 1]      

On the other hand, the code in the second example gives you this:

    ---
    |a|
    ---
     |
 +---+
 |   v
 |   [ref, 1]
 |     |
 +-----+

Here there is still only one array, and a still points to it. But now the first element in the array refers to the array itself.