Cloning an array with its content

问题:

I want to make a copy of an array, to modify the copy in-place, without affecting the original one. This code fails

a = [
  '462664',
  '669722',
  '297288',
  '796928',
  '584497',
  '357431'
]
b = a.clone
b.object_id == a.object_id # => false
a[1][2] = 'X'
a[1] #66X722
b[1] #66X722

The copy should be different than the object. Why does it act like if it were just a reference?

回答1:

You need to do a deep copy of your array.

Here is the way to do it

Marshal.load(Marshal.dump(a))

This is because you are cloning the array but not the elements inside. So the array object is different but the elements it contains are the same instances. You could, for example, also do a.each{|e| b << e.dup} for your case

回答2:

Instead of calling clone on the array itself, you can call it on each of the array's elements using map:

b = a.map(&:clone)

This works in the example stated in the question, because you get a new instance for each element in the array.

回答3:

Try this:

b = [] #create a new array 
b.replace(a) #replace the content of array b with the content from array a

At this point, these two arrays are references to different objects and content are the same.

回答4:

You can use #dup which creates a shallow copy of the object, meaning "the instance variables of object are copied, but not the objects they reference." For instance:

a = [1, 2, 3]

b = a.dup

b # => [1, 2, 3]

Source: https://ruby-doc.org/core-2.5.3/Object.html#method-i-dup

Edit: Listen to Paul below me. I misunderstood the question.

回答5:

Not sure , if this is answered anywhere else. Tried searching but no success.

Try this

current_array =["a", "b","c","d"]
new_array = current_array[0 .. current_array.length]
new_array[0] ="cool"

output of new_array
 "cool","b","c","d"

Hope this helps.