Ruby - local variable gets modified when changing instance variable

You have Array with Arrays, so you dup the first array but, inside object point to the same instance. In this case you just modify the same source.

like here:

arr = [[1, 2, 3]]
arr2 = arr.dup

arr2[0] << 1

p arr
# => [[1, 2, 3, 1]]
p arr2
# => [[1, 2, 3, 1]]

So you must use dup for all array instance like this.

arr = [[1, 2, 3]]
arr3 = arr.map(&:dup)
arr3[0] << 1

p arr
# => [[1, 2, 3]]
p arr3
# => [[1, 2, 3, 1]]

In your case use this map.

class Test
  def initialize
    @board = [[1,2],[3,4], [5,6]]
  end

  def modify
    temp = @board.map(&:dup) # dup all

    print 'temp: ';p temp
    print '@board: ';p @board

    @board.each do |x|
      x << "x"
    end

    print "\ntemp: ";p temp
    print '@board: ';p @board
  end
end

x = Test.new
x.modify
# temp: [[1, 2], [3, 4], [5, 6]]
# @board: [[1, 2], [3, 4], [5, 6]]
# 
# temp: [[1, 2], [3, 4], [5, 6]]
# @board: [[1, 2, "x"], [3, 4, "x"], [5, 6, "x"]]

The reason is clone and dup produce a shallow copy. Thus the object ids of the inner arrays are still the same: those are the same arrays.

What you need is a deep clone. There is no built-in method in the standard library able to do that.

From the docs:

dup produces a shallow copy of obj—the instance variables of obj are copied, but not the objects they reference.

That said: You will need to make a deep-copy of your array.

When you are using the ActiveSupport gem (Rails) you can use its deep_dup method instead of just calling dup:

temp = @board.deep_dup

Without gems mashaling might be a easy solution to deep dup almost everything, without knowing about the internals of an object:

temp = Marshal.load(Marshal.dump(@board))  

You can add a deep_dup method for nested arrays:

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

# To handle the exception when deepest array contains numeric value
class Numeric
  def deep_dup
    self
  end
end

class Test
    def initialize
        @board = [[1,2], [3,4], [5,6]]
    end

    def modify
        temp = @board.deep_dup
        ...
    end
end

x = Test.new
x.modify

# temp: [[1, 2], [3, 4], [5, 6]]
# @board: [[1, 2], [3, 4], [5, 6]]

# temp: [[1, 2], [3, 4], [5, 6]]
# @board: [[1, 2, "x"], [3, 4, "x"], [5, 6, "x"]]