Ruby-更改实例变量时修改本地变量 [英] Ruby - local variable gets modified when changing instance variable

问题描述

temp 获取 @ board.dup ，并修改 @board 数组.但是， temp 也会被修改！我已经尝试阅读所有相关文档，但仍然找不到解释.

temp gets @board.dup, and @board array is modified. However, temp gets modified as well! I have tried reading all the related documentations but still couldn't figure out an explanation.

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

    def modify
        temp = @board.dup #Also tried .clone

        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

输出:

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

temp: [[1, 2, "x"], [3, 4, "x"], [5, 6, "x"]] # <= Why did it change?
@board: [[1, 2, "x"], [3, 4, "x"], [5, 6, "x"]]

我该怎么做才能确保温度不被修改?

What can I do to ensure temp doesn't get modified?

推荐答案

您拥有带有数组的数组，因此您将第一个数组复制了，但是内部对象指向了同一实例.在这种情况下，您只需修改相同的源即可.

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.

喜欢这里:

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

arr2[0] << 1

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

因此，对于所有这样的数组实例，必须使用 dup .

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]]

在您的情况下，请使用此 map .

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"]]

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