Ruby/Rails 循环中的魔术第一个和最后一个指示器？

Magic First and Last Indicator in a Loop in Ruby/Rails?

提问by aronchick

Ruby/Rails does lots of cool stuff when it comes to sugar for basic things, and I think there's a very common scenario that I was wondering if anyone has done a helper or something similar for.

Ruby/Rails 在基本事物的糖方面做了很多很酷的事情，我认为有一个非常常见的场景，我想知道是否有人做过助手或类似的事情。

   a = Array.new(5, 1)

   a.each_with_index do |x, i|
     if i == 0
       print x+1
     elsif i == (a.length - 1)
       print x*10
     else
        print x
     end
   end

Pardon the ugliness, but this gets at what one might want... is there a ruby way to do something to the first and last of a loop?

请原谅丑陋，但这正是人们可能想要的......是否有一种红宝石方法可以对循环的第一个和最后一个执行某些操作？

[EDIT]I think ideally this would be an extension on Array with parameters (array instance, all elements function, first elements function, last elements function)... but I'm open to other thoughts.

[编辑]我认为理想情况下这将是对带参数的 Array 的扩展（数组实例、所有元素函数、第一个元素函数、最后一个元素函数）......但我对其他想法持开放态度。

采纳答案by Matchu

You could grab the first and last elements and process them differently, if you like.

如果您愿意，您可以获取第一个和最后一个元素并以不同的方式处理它们。

first = array.shift
last = array.pop
process_first_one
array.each { |x| process_middle_bits }
process_last_one

回答by sepp2k

If the code for the first and last iteration has nothing in common with the code for the other iterations, you could also do:

如果第一次和最后一次迭代的代码与其他迭代的代码没有任何共同之处，您也可以这样做：

do_something( a.first )
a[1..-2].each do |x|
  do_something_else( x )
end
do_something_else_else( a.last )

If the different cases have some code in common, your way is fine.

如果不同的情况有一些共同的代码，你的方式就可以了。

回答by Wayne Conrad

What if you could do this?

如果你能做到这一点呢？

%w(a b c d).each.with_position do |e, position|
  p [e, position]    # => ["a", :first]
                     # => ["b", :middle]
                     # => ["c", :middle]
                     # => ["d", :last]
end

Or this?

或这个？

%w(a, b, c, d).each_with_index.with_position do |(e, index), position|
  p [e, index, position]    # => ["a,", 0, :first]
                            # => ["b,", 1, :middle]
                            # => ["c,", 2, :middle]
                            # => ["d", 3, :last]
end

In MRI >= 1.8.7, all it takes is this monkey-patch:

在 MRI >= 1.8.7 中，只需要这个猴子补丁：

class Enumerable::Enumerator

  def with_position(&block)
    state = :init
    e = nil
    begin
      e_last = e
      e = self.next
      case state
      when :init
        state = :first
      when :first
        block.call(e_last, :first)
        state = :middle
      when :middle
        block.call(e_last, :middle)
      end
    rescue StopIteration
      case state
      when :first
        block.call(e_last, :first)
      when :middle
        block.call(e_last, :last)
      end
      return
    end while true
  end

end

It's got a little state engine because it must look ahead one iteration.

它有一个小的状态引擎，因为它必须向前看一次迭代。

The trick is that each, each_with_index, &c. return an Enumerator if given no block. Enumerators do everything an Enumerable does and a bit more. But for us, the important thing is that we can monkey-patch Enumerator to add one more way to iterate, "wrapping" the existing iteration, whatever it is.

诀窍是每个，each_with_index，&c。如果没有给定块，则返回一个枚举器。Enumerators 能做 Enumerable 能做的一切，甚至更多。但对我们来说，重要的是我们可以对 Enumerator 进行猴子补丁，以添加另一种迭代方式，“包装”现有的迭代，无论它是什么。

回答by Wayne Conrad

Or a tiny little Domain Specific Language:

或者一个很小的领域特定语言：

a = [1, 2, 3, 4]

FirstMiddleLast.iterate(a) do
  first do |e|
    p [e, 'first']
  end
  middle do |e|
    p [e, 'middle']
  end
  last do |e|
    p [e, 'last']
  end
end

# => [1, "first"]
# => [2, "middle"]
# => [3, "middle"]
# => [4, "last"]

and the code that makes it go:

以及让它运行的代码：

class FirstMiddleLast

  def self.iterate(array, &block)
    fml = FirstMiddleLast.new(array)
    fml.instance_eval(&block)
    fml.iterate
  end

  attr_reader :first, :middle, :last

  def initialize(array)
    @array = array
  end

  def first(&block)
    @first = block
  end

  def middle(&block)
    @middle = block
  end

  def last(&block)
    @last = block
  end

  def iterate
    @first.call(@array.first) unless @array.empty?
    if @array.size > 1
      @array[1..-2].each do |e|
        @middle.call(e)
      end
      @last.call(@array.last)
    end
  end

end

I started thinking, "if only you could pass multiple blocks to a Ruby function, then you could have a slick and easy solution to this question." Then I realized that DSL's play little tricks that are almost like passing multiple blocks.

我开始想，“如果你能将多个块传递给一个 Ruby 函数，那么你就可以有一个巧妙而简单的解决方案来解决这个问题。” 然后我意识到 DSL 的小技巧几乎就像传递多个块一样。

回答by Nate

As many have pointed out, each_with_indexseems to be the key to this. I have this code block that I liked.

正如许多人指出的那样，这each_with_index似乎是关键。我有这个我喜欢的代码块。

array.each_with_index do |item,index|
  if index == 0
    # first item
  elsif index == array.length-1
    # last item
  else
    # middle items
  end
  # all items
end

Or

或者

array.each_with_index do |item,index|
  if index == 0
    # first item
  end
  # all items
  if index == array.length-1
    # last item
  end
end

Or by Array extensions

或者通过数组扩展

class Array

  def each_with_position
    array.each_with_index do |item,index|
      if index == 0
        yield item, :first
      elsif index == array.length-1
        yield item, :last
      else
        yield item, :middle
      end
    end
  end

  def each_with_index_and_position
    array.each_with_index do |item,index|
      if index == 0
        yield item, index, :first
      elsif index == array.length-1
        yield item, index, :last
      else
        yield item, index, :middle
      end
    end
  end

  def each_with_position_and_index
    array.each_with_index do |item,index|
      if index == 0
        yield item, :first, index
      elsif index == array.length-1
        yield item, :last, index
      else
        yield item, :middle, index
      end
    end
  end

end

回答by clord

If you are willing to add some boilerplate, you can add something like this to the array class:

如果您愿意添加一些样板，您可以在数组类中添加如下内容：

class Array
  def each_fl
    each_with_index do |x,i|
      yield [i==0 ? :first : (i==length-1 ? :last : :inner), x]
    end
  end
end

and then anywhere you need to, you get the following syntax:

然后在任何你需要的地方，你会得到以下语法：

[1,2,3,4].each_fl do |t,x|
  case t
    when :first
      puts "first: #{x}"
    when :last
      puts "last: #{x}"
    else
      puts "otherwise: #{x}"
  end
end

for the following output:

对于以下输出：

first: 1
otherwise: 2
otherwise: 3
last: 4

回答by John Feminella

There's no "do this the (first|last) time" syntax in Ruby. But if you're looking for succinctness, you could do this:

Ruby 中没有“在（第一次|最后一次）执行此操作”的语法。但如果你正在寻找简洁，你可以这样做：

a.each_with_index do |x, i|
  print (i > 0 ? (i == a.length - 1 ? x*10 : x) : x+1)
end

The result is what you'd expect:

结果是您所期望的：

irb(main):001:0> a = Array.new(5,1)
=> [1, 1, 1, 1, 1]
irb(main):002:0> a.each_with_index do |x,i|
irb(main):003:1*   puts (i > 0 ? (i == a.length - 1 ? x*10 : x) : x+1)
irb(main):004:1> end
2
1
1
1
10

回答by Andrew Grimm

Interesting question, and one I've thought a bit about as well.

有趣的问题，我也想过一个问题。

I think you'd have to create three different blocks/procs/whatever they're called, and then create a method that calls the correct block/proc/whatever. (Sorry for the vagueness - I'm not yet a black belt metaprogrammer) [Edit: however, I've copied from someone who is at the bottom)

我认为您必须创建三个不同的块/过程/它们被调用的任何东西，然后创建一个调用正确块/过程/任何东西的方法。（抱歉含糊不清 - 我还不是黑带元程序员）[编辑：但是，我是从底层的人那里复制的）

class FancyArray
  def initialize(array)
    @boring_array = array
    @first_code = nil
    @main_code = nil
    @last_code = nil
  end

  def set_first_code(&code)
    @first_code = code
  end

  def set_main_code(&code)
    @main_code = code
  end

  def set_last_code(&code)
    @last_code = code
  end

  def run_fancy_loop
    @boring_array.each_with_index do |item, i|
      case i
      when 0 then @first_code.call(item)
      when @boring_array.size - 1 then @last_code.call(item)
      else @main_code.call(item)
      end
    end
  end
end

fancy_array = FancyArray.new(["Matti Nykanen", "Erik Johnsen", "Michael Edwards"])
fancy_array.set_first_code {|item| puts "#{item} came first in ski jumping at the 1988 Winter Olympics"}
fancy_array.set_main_code {|item| puts "#{item} did not come first or last in ski jumping at the 1988 Winter Olympics"}
fancy_array.set_last_code {|item| puts "#{item} came last in ski jumping at the 1988 Winter Olympics"}
fancy_array.run_fancy_loop

produces

产生

Matti Nykanen came first in ski jumping at the 1988 Winter Olympics
Erik Johnsen did not come first or last in ski jumping at the 1988 Winter Olympics
Michael Edwards came last in ski jumping at the 1988 Winter Olympics

Edit: Svante's answer(with molf's suggestion) to a related question shows how to pass in multiple code blocks to a single method:

编辑：Svante对相关问题的回答（有 molf 的建议）显示了如何将多个代码块传递给单个方法：

class FancierArray < Array
  def each_with_first_last(first_code, main_code, last_code)
    each_with_index do |item, i|
      case i
        when 0 then first_code.call(item)
        when size - 1 then last_code.call(item)
        else main_code.call(item)
      end
    end
  end
end

fancier_array = FancierArray.new(["Matti Nykanen", "Erik Johnsen", "Michael Edwards"])
fancier_array.each_with_first_last(
  lambda {|person| puts "#{person} came first in ski jumping at the 1988 Winter Olympics"},
  lambda {|person| puts "#{person} did not come first or last in ski jumping at the 1988 Winter Olympics"},
  lambda {|person| puts "#{person} came last in ski jumping at the 1988 Winter Olympics"})

回答by 2called-chaos

I needed this functionality from time to time, so I crafted a little class for that purpose.

我不时需要这个功能，所以我为此制作了一个小类。

The latest version is at: https://gist.github.com/3823837

最新版本在：https: //gist.github.com/3823837

Sample:

样本：

("a".."m").to_a.each_pos do |e|
  puts "Char\tfirst?\tlast?\tprev\tnext\twrapped?\tindex\tposition" if e.first?
  print "#{e.item}\t"
  print "#{e.first?}\t"
  print "#{e.last?}\t"
  print "#{e.prev}\t"
  print "#{e.next}\t"
  print "#{e.wrapped?}\t\t"
  print "#{e.index}\t"
  puts  "#{e.position}\t"
end

# Char  first?  last?  prev  next  wrapped?  index  position
# a     true    false        b     false     0      1
# b     false   false  a     c     true      1      2
# c     false   false  b     d     true      2      3
# d     false   false  c     e     true      3      4
# e     false   false  d     f     true      4      5
# f     false   false  e     g     true      5      6
# g     false   false  f     h     true      6      7
# h     false   false  g     i     true      7      8
# i     false   false  h     j     true      8      9
# j     false   false  i     k     true      9      10
# k     false   false  j     l     true      10     11
# l     false   false  k     m     true      11     12
# m     false   true   l           false     12     13



{
  a: "0",
  b: "1",
  c: "2",
  d: "3",
  e: "4",
  f: "5",
  g: "6",
  h: "7",
  i: "8",
  j: "9",
  k: "10",
  l: "11",
  m: "12",
}.each_pos do |(k, v), e|
  puts "KV\tChar\t\tfirst?\tlast?\tprev\t\tnext\t\twrapped?\tindex\tposition" if e.first?
  print "#{k} => #{v}\t"
  print "#{e.item}\t"
  print "#{e.first?}\t"
  print "#{e.last?}\t"
  print "#{e.prev || "\t"}\t"
  print "#{e.next || "\t"}\t"
  print "#{e.wrapped?}\t\t"
  print "#{e.index}\t"
  puts  "#{e.position}\t"
end

# KV      Char        first?  last?   prev        next        wrapped?  index position
# a => 0  [:a, "0"]   true    false               [:b, "1"]   false     0     1
# b => 1  [:b, "1"]   false   false   [:a, "0"]   [:c, "2"]   true      1     2
# c => 2  [:c, "2"]   false   false   [:b, "1"]   [:d, "3"]   true      2     3
# d => 3  [:d, "3"]   false   false   [:c, "2"]   [:e, "4"]   true      3     4
# e => 4  [:e, "4"]   false   false   [:d, "3"]   [:f, "5"]   true      4     5
# f => 5  [:f, "5"]   false   false   [:e, "4"]   [:g, "6"]   true      5     6
# g => 6  [:g, "6"]   false   false   [:f, "5"]   [:h, "7"]   true      6     7
# h => 7  [:h, "7"]   false   false   [:g, "6"]   [:i, "8"]   true      7     8
# i => 8  [:i, "8"]   false   false   [:h, "7"]   [:j, "9"]   true      8     9
# j => 9  [:j, "9"]   false   false   [:i, "8"]   [:k, "10"]  true      9     10
# k => 10 [:k, "10"]  false   false   [:j, "9"]   [:l, "11"]  true      10    11
# l => 11 [:l, "11"]  false   false   [:k, "10"]  [:m, "12"]  true      11    12
# m => 12 [:m, "12"]  false   true    [:l, "11"]              false     12    13

Actual class:

实际班级：

module Enumerable
  # your each_with_position method
  def each_pos &block
    EachWithPosition.each(self, &block)
  end
end

class EachWithPosition
  attr_reader :index

  class << self
    def each *a, &b
      handler = self.new(*a, :each, &b)
    end
  end

  def initialize collection, method, &block
    @index = 0
    @item, @prev, @next = nil
    @collection = collection
    @callback = block
    self.send(method)
  end

  def count
    @collection.count
  end
  alias_method :length, :count
  alias_method :size, :count

  def rest
    count - position
  end

  def first?
    @index == 0
  end

  def last?
    @index == (count - 1)
  end

  def wrapped?
    !first? && !last?
  end
  alias_method :inner?, :wrapped?

  def position
    @index + 1
  end

  def prev
    @prev
  end

  def next
    @next
  end

  def current
    @item
  end
  alias_method :item, :current
  alias_method :value, :current

  def call
    if @callback.arity == 1
      @callback.call(self)
    else
      @callback.call(@item, self)
    end
  end

  def each
    @collection.each_cons(2) do |e, n|
      @prev = @item
      @item = e
      @next = n

      self.call
      @index += 1

      # fix cons slice behaviour
      if last?
        @prev, @item, @next = @item, @next, nil
        self.call
        @index += 1
      end
    end
  end
end

回答by Sagiv Ofek

KISS

吻

arr.each.with_index do |obj, index|
  p 'first' if index == 0        
  p 'last' if index == arr.count-1                  
end