Duplicating a Ruby array of strings

Question

arr = ["red","green","yellow"]

arr2 = arr.clone
arr2[0].replace("blue")

puts arr.inspect
puts arr2.inspect

produces:

["blue", "green", "yellow"]
["blue", "green", "yellow"]

Is there anyway to do a deep copy of an array of strings, other than using Marshal as i understand that is a hack.

I could do:

arr2 = []
arr.each do |e|
  arr2 << e.clone
end

but it doesn't seem very elegant, or efficient.

Thanks

Rheumatism

you can do shorter inline block: arr.each{|e| arr2 << e.dup} – Morty

Answer 1

Your second solution can be shortened to arr2 = arr.map do |e| e.dup end (unless you actually need the behaviour of clone, it's recommended to use dup instead).

Other than that your two solutions are basically the standard solutions to perform a deep copy (though the second version is only one-level deep (i.e. if you use it on an array of arrays of strings, you can still mutate the strings)). There isn't really a nicer way.

Edit: Here's a recursive deep_dup method that works with arbitrarily nested arrays:

class Array
  def deep_dup
    map {|x| x.deep_dup}
  end
end

class Object
  def deep_dup
    dup
  end
end

class Numeric
  # We need this because number.dup throws an exception
  # We also need the same definition for Symbol, TrueClass and FalseClass
  def deep_dup
    self
  end
end

You might also want to define deep_dup for other containers (like Hash), otherwise you'll still get a shallow copy for those.

Levalloisian

Thanks for your answer sepp2k, so if you have a nested array, the only way is to use Marshal? – Rheumatism

@Jon: No you could also define a recursive deep_dup method (see my edit), but using Marshal is usually easier. – Levalloisian

Can't you avoid patching Numeric etc. by defining deep_dup in Object to do this?: respond_to?(:dup) ? dup : self – Adelric

@Lars: No, because Numerics do respond to dup. They just respond by throwing an exception. – Levalloisian

@sepp2k: Bummer. Then something like [Numeric, Symbol, TrueClass, FalseClass].include?(self.class) ? self : dup; ought to do the trick. – Adelric

@LarsHaugseth: It does if you replace include?(self.class) with any? {|c| self.is_a? c} or if you add all of the subclasses of Numeric to the array. – Levalloisian

There is a problem with DUP if your array has FixNums like integer - can't dup Fixnum #1964643 https://mcmap.net/q/1013710/-ruby-can-39-t-dup-fixnum – Enzymolysis

@Enzymolysis That's why I defined deep_dup to return self rather than calling dup for numbers. – Levalloisian

Answer 2

I am in a similar situation and very concerned about speed. The fastest way for me was to make use of map{&:clone}

So try this:

pry(main)> a = (10000..1000000).to_a.shuffle.map(&:to_s)
pry(main)> Benchmark.ms { b = a.deep_dup }                                                                                     
=> 660.7760030310601
pry(main)> Benchmark.ms { b = a.join("--!--").split("--!--") }
=> 605.0828141160309
pry(main)> Benchmark.ms { b = a.map(&:clone) }
=> 450.8283680770546

Whereunto

Answer 3

I recommend your initial idea, but written slightly more concisely:

arr = ["red","green","yellow"]
arr2 = arr.inject([]) { |a,element| a << element.dup }

Wales

There is a problem with DUP if your array has FixNums like integer - can't dup Fixnum #1964643 https://mcmap.net/q/1013710/-ruby-can-39-t-dup-fixnum – Enzymolysis

Answer 4

You can make a deep copy of array a by following code:

 Marshal.load(Marshal.dump(a))

Transported

Answer 5

It looks so simple.. Just run the below code:

a = [1,2,3]
b = [].replace(a)

b[1] = 5

puts a
puts b

Run above code and you will notice the difference. Cheers !

Begorra

Answer 6

You can use this hack:

arr1 = %w{ red green blue }
arr2 = arr1.join("--!--").split("--!--")

But it is just for fun :)

arr2[0].replace("lol")
p arr1
#=> ["red", "green", "blue"]
p arr2
#=> ["lol", "green", "blue"]

And it will work only for 1 level arrays

Morty

It will also only work if the array only contains strings and none of the strings contain "--!--" as a substring. – Levalloisian

sepp2k, yeap, it's just hack for one purpose only :) like tetra pack – Morty