Why does this usage of until in Ruby not work, while another usage does?

问题

The code below gives an error:

def bubble_sort(arr)
  until arr == arr.sort do
    (arr.count - 1).times do |i|
      (arr[i], arr[i + 1] = arr[i + 1], arr[i]) if (arr[i] > arr[i + 1])
    end
  end 

  arr
end

(eval):433: (eval):433: compile error (SyntaxError)

(eval):426: syntax error, unexpected kDO_COND, expecting kEND (arr.count - 1).times do |i| ^

(eval):433: syntax error, unexpected kEND, expecting $end

But another variation of it below passes successfully. Why?

def bubble_sort(arr)
  begin
    (arr.count - 1).times do |i|
      (arr[i], arr[i + 1] = arr[i + 1], arr[i]) if (arr[i] > arr[i + 1])
    end
  end until arr == arr.sort

  arr
end

回答1:

After a bit of debugging, this looks like a bug in irb on 1.8.7 (the version that labs.codecademy.com uses). Locally, when installing Ruby 1.8.7-p374 I see the same thing. However, if I run the code as a normal Ruby file, it works just fine.

However, Ruby 1.8.7 is officially retired, and it seems to me like the Codecademy Labs site is no longer maintained, either, so I recommend either learning Ruby with the "full" Codecademy site, or installing a more recent version of Ruby (I recommend 2.0.0) locally.

回答2:

I don't know why, but if you change the first code to the following it'll work out:

def bubble_sort(arr)
  total = (arr.count - 1)
  until arr == arr.sort do
    total.times do |i|
      (arr[i], arr[i + 1] = arr[i + 1], arr[i]) if (arr[i] > arr[i + 1])
    end
  end 

  arr
end

I've tested it with your link, so I'm guessing maybe it isn't properly configured, cause when I first added that total in until, it turned into an infinite loop. But someone has to check it against ruby, it maybe a bug.

回答3:

In Ruby, the until block doesn't use do, so you should do something like this:

def bubble_sort(arr)
  until arr == arr.sort
    (arr.count - 1).times do |i|
      (arr[i], arr[i + 1] = arr[i + 1], arr[i]) if (arr[i] > arr[i + 1])
    end
  end 

  arr
end

Edit: Expanded the code so it includes the algorithm from the original question. This code works for me on ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-darwin12.4.0].

来源：https://stackoverflow.com/questions/17754479/why-does-this-usage-of-until-in-ruby-not-work-while-another-usage-does

标签

ruby

block