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Suppose I declare arrays as follows: 

array1 = [1,2,3,4,5] array2 = array1

The object ID of both arrays are the same:

array1.object_id = 118945940 array2.object_id = 118945940

When I insert an element in an array as follows,

array1 << 10

the result is

array1 = [1, 2, 3, 4, 5, 10] array2 = [1, 2, 3, 4, 5, 10]

But when I add new array into array array1,

array1 = array1 + [11,12] array1 = [1,2,3,4,5,10,11,12] array2 = [1,2,3,4,5,10]

the object ID of both arrays have changed.

How does << work?

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2 Answers 2

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I believe that the confusion here is due to the fact that concatenating two arrays actually creates a new array (ie: a new object).

array1 + array2

This will create a new array with the contents of both arrays inside. So the object id will change because it is a different object.

When you use the << operator it just adds an element to the array - ruby doesn't create a new object and hence both arrays (which share an object id) get the new element.

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5 Comments

So is << like array.append?
@Arc676 - Yes. This is my intention.
This post stackoverflow.com/questions/1801516/…, while not directly related to your question contains useful and relevant information to your issue.
What do you mean by "there is no need to create a new object"? << just doesn't create a new object.
@sawa - perhaps the wrong choice of words... What I mean is that appending an element to an array does not create a new object. You don't need to use code/methods that creates a new object.
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array is a variable containing a reference (pointer, handle, ID - whatever you want to call it) to the actual array (the instance of class Array).

array (variable) -------------------> object (instance of class Array)

What does your code do? Let's see:

array << value

This is exactly the same as calling a method: array.append(value). No new instance of Array is created. You are not assigning a new value to the variable array. The actual array object is of course changing, but not its reference.

array1 = array1 + array2

The right side of the assignment, array1 + array2, must necessarily create a new Array instance. This is easy to see - the code intuitively should not modify array1 or array2. The reference to this new array is then stored in the variable array1.

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