arrays - 由于另一个数组，Ruby 空数组在迭代中更改值

Question

一个空数组定义在 block 外，在 block 内，它被分配给一个新变量。如果新数组发生变化，空数组也会修改它的值。为什么？

# THIS CODE CHECK WHICH LETTERS IN klas APPEAR IN docs
klas = ["a", "b", "c"]

docs = [[1, "a"], [2, "a"], [3, "b"], [4, "b"], [5, "c"], [6, "c"]]

output = []
empty_array = Array.new(klas.size) { 0 } # EMPTY ARRAY DEFINED HERE

docs.each do |doc|
    puts empty_array.inspect
    puts (output_row = empty_array).inspect # EMPTY ARRAY ASSIGNED TO ANOTHER

    # FIND INDEX OF THE LETTER IN klas FROM docs. ASSIGN 1.
    output_row[klas.index(doc[1])] = 1 # PROBLEM! THIS ALSO CHANGES THE EMPTY ARRAY VALUES
    output << output_row
end

CONSOLE OUTPUT 显示空数组根据另一个数组更改其值

###
empty_array is [0, 0, 0]
output_row is [0, 0, 0]
---
###
empty_array is [1, 0, 0]
output_row is [1, 0, 0]
---
###
empty_array is [1, 0, 0]
output_row is [1, 0, 0]
---
###
empty_array is [1, 1, 0]
output_row is [1, 1, 0]
---
###
empty_array is [1, 1, 0]
output_row is [1, 1, 0]
---
###
empty_array is [1, 1, 1]
output_row is [1, 1, 1]
---

# INCORRECT output IS
=> [[1, 1, 1], [1, 1, 1], [1, 1, 1], [1, 1, 1], [1, 1, 1], [1, 1, 1]]
# SHOULD BE
=> [[1, 0, 0], [1, 0, 0], [0, 1, 0], [0, 1, 0], [0, 0, 1], [0, 0, 1]]

但如果空数组在 block 中定义并分配给一个新变量，它会按预期工作。

docs.each do |doc|
    empty_array = Array.new(klas.size) { 0 } # THIS MAKES SURE empty_array stays with zero values
    output_row = empty_array    
    output_row[klas.index(doc[1])] = 1
    output << output_row
end

CORRECT output IS
=> [[1, 0, 0], [1, 0, 0], [0, 1, 0], [0, 1, 0], [0, 0, 1], [0, 0, 1]]

为什么空数组在 block 外时修改它的值？不管另一个数组改变它的值，它不应该保持不变吗？

Answer 1

通过使用

output_row = empty_array

您没有复制空数组。您正在创建对 empty_array 中引用的相同底层数组的引用。

您可以创建阵列的克隆或副本。所以使用:

output_row = empty_array.dup

这将创建一个新数组，它是 empty_array 的副本，参见 http://ruby-doc.org/core-2.4.1/Object.html#method-i-dup

关于示例发生的事情的更完整解释:

a = [1] => [1] # is creating a new array 'x'
               # a is referencing this array, a is not the array itself!

b = a   => [1] # b now references the array 'x'

a = [2] => [2] # is creating a new array 'y', a is referencing this new array
               # the reference of b is not changing

b       => [1] # b still pointing to array 'x'

所以对于 b = a 你只是告诉他们引用相同的数组 x，但是 b 不是对 的引用一个。