将数组传递给 Array#each，内部使用引用或值

Question

array1 = [0, 1]

# CASE 1:  
[array1].each do |arr| 
  arr = [3, 4] 
end

=> [0, 1]

# CASE 2:
[array1].each do |arr| 
  arr.delete_if { |ele| ele == 0 }
end

=> [[1]]

我认为 ruby​​ 总是通过引用传递数组。为什么在 CASE 1 中它没有改变数组的值，但在 CASE 2 中却改变了它？谢谢。

Answer 1

在第一种情况下，您所做的只是更改 arr 指向的对象 - 您实际上并未修改原始对象。这可以通过以下脚本来证明：

# Given our test value...
test_array = [1, 2]

# we can verify the values and the object_ids
puts "Value of `test_array`: #{test_array.inspect}"
puts "Object_id of `test_array`: #{test_array.object_id}"

# now, let's put it in a container and run it through a block
@array_container = [test_array]

@array_container.each do |arr|

  # Just to prove that `arr` points at test_array
  puts "Object_id of `arr`: #{arr.object_id}"

  # and that it's the same as the first element in our container
  puts "@container.first.object_id: #{@array_container.first.object_id}"

  # but, when we re-assign our block variable
  arr = [3, 4]

  # we get different values
  puts "Object_id of `arr`: #{arr.object_id}"
  puts "@container.first.object_id: #{@array_container.first.object_id}"
end

哪些输出...

Value of `test_array`: [1, 2]
Object_id of `test_array` : 2150710260

Object_id of `arr`        : 2150710260
@container.first.object_id: 2150710260

Object_id of `arr`        : 2150708040
@container.first.object_id: 2150710260

那么案例 2 有何不同？在情况 2 中，您实际上调用了一个自毁性方法，该方法将对 arr 引用的原始对象进行更改

Answer 2

我认为你的问题与范围有关。 do-end 构造实际上是一个新块，因此当您声明 arr = [3, 4] 时，您正在实例化一个新的 arr = [3, 4 ] 在该块内。而在情况 2 中，您直接修改引用。在案例 1 中做同样的事情会是这样的：

[array1].each do |arr| 
  arr.each_index do |i|
    arr[i]+=3
  end 
end

=> [[3,4]]

Answer 3

.each 只是依次调用该块及其所调用的集合的每个元素（在本例中为 [array1]）。传递参数只是将其分配给参数。您的示例可以简化为以下内容：

# CASE 1:  
array1 = [0, 1]
arr = array1
arr = [3, 4]
array1

=> [0, 1]

# CASE 2:
array1 = [0, 1]
arr = array1
arr.delete_if { |ele| ele == 0 }
array1

=> [1]