arrays - 如何在 Ruby 中的数组上使用 for 循环

Question

我需要使用 for 循环从 more_stuff 数组中取出最后四个字符串。

这是我的代码:

# assigns string Apples Oranges Crows Telephone Light Sugar to variable 
ten_things
ten_things = "Apples Oranges Crows Telephone Light Sugar"

# prints Wait there are not ten things on that list. Let's fix that.
puts "Wait there are not 10 things in that list. Let's fix that."

# first seperates string ten_things into an array at space character, then 
# assigns the new array to variable stuff
stuff = ten_things.split(' ')
# assigns array to more stuff with strings, Day, Night, Song, Frisbee, Girl, 
# and Boy
more_stuff = ["Day", "Night", "Song", "Frisbee", "Corn", "Banana", "Girl", 
"Boy"]

# using math to make sure there's 10 items

# assigns fifth through eighth elements in array more_stuff to array stuff_3
stuff_3 = more_stuff[4..8]
# prints array stuff_3 in brackets
puts "#{stuff_3}"
# for all strings in stuff_3
stuff_3.each do |stuff_3|
  # pops the last item off of stuff_3
  next_one = stuff_3.pop
  # puts adding (next_one)
  puts "Adding #{next_one}"
  # adds (next_one) to array stuff
  stuff.push(next_one)
# ends for loop
end

此外，这是我从 Powershell 运行它时出现的错误:

Wait there are not 10 things in that list. Let's fix that.
["Corn", "Banana", "Girl", "Boy"]
ex38for.rb:17:in `block in <main>': undefined method `pop' for "Corn":String 
(NoMethodError)
    from ex38for.rb:16:in `each'
    from ex38for.rb:16:in `<main>'

我对 for 循环的工作方式感到困惑，特别是 each 以及在数组命令中放置内容的位置。

Answer 1

pop 的作用是获取数组的最后一个元素或值(当使用时未指定元素数):

p stuff_3.pop
# => "Boy"
p stuff_3.pop(2)
# => ["Banana", "Girl"]

但在您的情况下，您正尝试将 pop 与主数组内的元素一起使用。

如果您在 each 方法之外检查它:

puts stuff_3.pop
# => Boy

然后将打印 Boy，因为它是 stuff_3 数组中的最后一个元素，您将其声明为 more_stuff[4..8]:

stuff_3 = more_stuff[4..8]
p stuff_3
# => ["Corn", "Banana", "Girl", "Boy"]

但是，当您执行 stuff_3.each do |stuff_3| 时，您使用相同的名称 stuff_3 来访问该数组中的每个元素，该数组具有相同的姓名。所以你会得到 undefined method 'pop' for "Corn":String 错误，因为 pop 正在等待 array 并且如果您遍历 stuff_3 中的每个元素，您将获得 String 元素。

一个可能的解决方案是，当您将 each 与 stuff_3 数组一起使用时，您可以使用不同的名称来访问元素，也许就像 stuffs，这将为您提供“Adding Boy”和“Adding Girl”。

或者任何引用 stuff_3 中的元素的词都可以与这个不同，因为你没有访问这些元素，你也可以使用 _ 来指定它们未被使用:

stuff_3.each do |_|
  next_one = stuff_3.pop
  puts "Adding #{next_one}"
  stuff.push(next_one)
end

在推送元素之前，这将是stuff:

["Apples", "Oranges", "Crows", "Telephone", "Light", "Sugar"]

之后:

["Apples", "Oranges", "Crows", "Telephone", "Light", "Sugar", "Boy", "Girl"]