[英]Magic First and Last Indicator in a Loop in Ruby/Rails?
Ruby / Rails在谈到基本用品的糖时会做很多很酷的事情,我认为有一种非常常见的情况,我想知道是否有人做过帮手或类似的东西。
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
原谅丑陋,但这可以达到人们想要的......是否有一种红宝石的方式来对循环的第一个和最后一个做一些事情?
[编辑]我认为理想情况下这将是带参数的数组扩展(数组实例,所有元素函数,第一个元素函数,最后元素函数)......但我对其他想法持开放态度。
如果您愿意,可以抓取第一个和最后一个元素并以不同方式处理它们。
first = array.shift
last = array.pop
process_first_one
array.each { |x| process_middle_bits }
process_last_one
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,您还可以执行以下操作:
do_something( a.first )
a[1..-2].each do |x|
do_something_else( x )
end
do_something_else_else( a.last )
如果不同的情况有一些共同的代码,你的方式很好。
如果你能这样做怎么办?
%w(a b c d).each.with_position do |e, position|
p [e, position] # => ["a", :first]
# => ["b", :middle]
# => ["c", :middle]
# => ["d", :last]
end
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
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
它有一个小的状态引擎,因为它必须向前看一次迭代。
诀窍是each,each_with_index,&c。 如果没有阻止,则返回枚举器。 枚举器可以完成Enumerable所做的所有事情。 但对我们来说,重要的是我们可以通过猴子修补Enumerator来添加一种迭代方式,“包装”现有的迭代,无论它是什么。
或者一点点领域特定语言:
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"]
和它的代码:
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
我开始思考,“如果只有你可以将多个块传递给Ruby函数,那么你可以对这个问题有一个灵活而简单的解决方案。” 然后我意识到DSL的玩法几乎就像传递多个块一样。
正如许多人所指出的, 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
要么
array.each_with_index do |item,index|
if index == 0
# first item
end
# all items
if index == array.length-1
# last item
end
end
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
如果您愿意添加一些样板文件,可以在数组类中添加以下内容:
class Array
def each_fl
each_with_index do |x,i|
yield [i==0 ? :first : (i==length-1 ? :last : :inner), x]
end
end
end
然后你需要的任何地方,你得到以下语法:
[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
对于以下输出:
first: 1
otherwise: 2
otherwise: 3
last: 4
在Ruby中没有“执行此操作(最后|最后)时间”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
结果就是你所期望的:
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
有趣的问题,我也想过一个问题。
我认为你必须创建三个不同的块/ procs /无论它们被调用,然后创建一个调用正确的块/ proc /的方法。 (抱歉模糊不清 - 我还不是黑带元程序员)[ 编辑 :但是,我是从底部的人那里复制的)
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
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
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"})
我不时需要这个功能,所以我为此目的制作了一个小课程。
最新版本位于: https : //gist.github.com/3823837
样品:
("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
实际课程:
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
吻
arr.each.with_index do |obj, index|
p 'first' if index == 0
p 'last' if index == arr.count-1
end
我无法抗拒:)虽然我认为它不应该比这里的大多数其他答案慢得多,但这并没有针对性能进行调整。 这都是糖!
class Array
class EachDSL
attr_accessor :idx, :max
def initialize arr
self.max = arr.size
end
def pos
idx + 1
end
def inside? range
range.include? pos
end
def nth? i
pos == i
end
def first?
nth? 1
end
def middle?
not first? and not last?
end
def last?
nth? max
end
def inside range
yield if inside? range
end
def nth i
yield if nth? i
end
def first
yield if first?
end
def middle
yield if middle?
end
def last
yield if last?
end
end
def each2 &block
dsl = EachDSL.new self
each_with_index do |x,i|
dsl.idx = i
dsl.instance_exec x, &block
end
end
end
例1:
[1,2,3,4,5].each2 do |x|
puts "#{x} is first" if first?
puts "#{x} is third" if nth? 3
puts "#{x} is middle" if middle?
puts "#{x} is last" if last?
puts
end
# 1 is first
#
# 2 is middle
#
# 3 is third
# 3 is middle
#
# 4 is middle
#
# 5 is last
例2:
%w{some short simple words}.each2 do |x|
first do
puts "#{x} is first"
end
inside 2..3 do
puts "#{x} is second or third"
end
middle do
puts "#{x} is middle"
end
last do
puts "#{x} is last"
end
end
# some is first
# short is second or third
# short is middle
# simple is second or third
# simple is middle
# words is last
如果你不介意“最后”动作发生在中间的东西之前,那么这个猴子补丁:
class Array
def for_first
return self if empty?
yield(first)
self[1..-1]
end
def for_last
return self if empty?
yield(last)
self[0...-1]
end
end
允许这个:
%w(a b c d).for_first do |e|
p ['first', e]
end.for_last do |e|
p ['last', e]
end.each do |e|
p ['middle', e]
end
# => ["first", "a"]
# => ["last", "d"]
# => ["middle", "b"]
# => ["middle", "c"]
将数组划分为范围,其中每个范围内的元素应该表现不同。 将如此创建的每个范围映射到块。
class PartitionEnumerator
include RangeMaker
def initialize(array)
@array = array
@handlers = {}
end
def add(range, handler)
@handlers[range] = handler
end
def iterate
@handlers.each_pair do |range, handler|
@array[range].each { |value| puts handler.call(value) }
end
end
end
可以手动创建范围,但下面的这些帮助使它更容易:
module RangeMaker
def create_range(s)
last_index = @array.size - 1
indexes = (0..last_index)
return (indexes.first..indexes.first) if s == :first
return (indexes.second..indexes.second_last) if s == :middle
return (indexes.last..indexes.last) if s == :last
end
end
class Range
def second
self.first + 1
end
def second_last
self.last - 1
end
end
用法:
a = [1, 2, 3, 4, 5, 6]
e = PartitionEnumerator.new(a)
e.add(e.create_range(:first), Proc.new { |x| x + 1 } )
e.add(e.create_range(:middle), Proc.new { |x| x * 10 } )
e.add(e.create_range(:last), Proc.new { |x| x } )
e.iterate
我看到很多hacks非常接近,但都严重依赖于具有固定大小的给定迭代器而不是迭代器。 我还建议在迭代时保存前一个元素,以了解迭代的第一个/最后一个元素。
previous = {}
elements.each do |element|
unless previous.has_key?(:element)
# will only execute the first time
end
# normal each block here
previous[:element] = element
end
# the last element will be stored in previous[:element]
如果您知道数组中的项是唯一的(与此情况不同),您可以这样做:
a = [1,2,3,4,5]
a.each_with_index do |x, i|
if x == a.first
print x+1
elsif x == a.last
print x*10
else
print x
end
end
有时for循环只是你最好的选择
if(array.count > 0)
first= array[0]
#... do something with the first
cx = array.count -2 #so we skip the last record on a 0 based array
for x in 1..cx
middle = array[x]
#... do something to the middle
end
last = array[array.count-1]
#... do something with the last item.
end
我知道这个问题已经回答了,但是这个方法没有副作用,也没有检查13,14,15 ...... 10ththth,10,001th ...记录是第一个记录,还是最后一个记录。
以前的答案将在任何数据结构类中失败。
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.