Question

I was going through The Well Grounded Rubyist and got confused by the following example.

Suppose we have an array of strings:

    numbers = ["one", "two", "three"]

If I freeze this array, I can’t do the following:

    numbers[2] = "four"

That statement is a Runtime error, but this:

    numbers[2].replace("four")

is not.

The book explains that in the first of the last two statements, we are trying to access the array. That’s what I found confusing because I thought we are trying to access the third element of the array, which is a string object. And how is that different from the last statement?

Answer 1

It’s different because in the statement that works you are calling String#replace. As you might expect, a call to Array#replace will fail.

numbers.replace [1,2,3]
TypeError: can't modify frozen array

The object reference at any given array index might be arbitrarily complicated and it’s not the job of the frozen array to keep those objects from changing … it just wants to keep the array from changing. You can see this:

ree-1.8.7> numbers[2].object_id
 => 2149301040 
ree-1.8.7> numbers[2].replace "four"
 => "four" 
ree-1.8.7> numbers[2].object_id
 => 2149301040

numbers[2] has the same object_id after String#replace runs; the Array did not actually change.