如何在 Ruby 循环中的第一次迭代中采取不同的行动？
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How to act differently on first iteration in a Ruby loop?
提问by Panagiotis Panagi
I always use a counter to check for the first item (i==0) in a loop:
我总是使用计数器来检查i==0循环中的第一项 ( )：
i = 0
my_array.each do |item|
if i==0
# do something with the first item
end
# common stuff
i += 1
end
Is there a more elegant way to do this (perhaps a method)?
有没有更优雅的方法来做到这一点（也许是一种方法）？
回答by detunized
回答by Russell
Using each_with_index, as others have described, would work fine, but for the sake of variety here is another approach.
使用each_with_index，正如其他人所描述的，将做工精细，但对于不同的缘故这里是另一种方法。
If you want to do something specific for the first element only and something general for all elements including the first, you could do:
如果您只想为第一个元素做一些特定的事情，而对包括第一个元素在内的所有元素做一些通用的事情，您可以这样做：
# do something with my_array[0] or my_array.first
my_array.each do |e|
# do the same general thing to all elements
end
But if you want to not do the general thing with the first element you could do:
但是如果你不想用第一个元素做一般的事情，你可以这样做：
# do something with my_array[0] or my_array.first
my_array.drop(1).each do |e|
# do the same general thing to all elements except the first
end
回答by undur_gongor
What fits best is depending on the situation.
什么最合适取决于具体情况。
Another option (if you know your array is not empty):
另一种选择（如果您知道您的数组不为空）：
# treat the first element (my_array.first)
my_array.each do | item |
# do the common_stuff
end
回答by Toby Hede
Arrays have an "each_with_index" method which is handy for this situation:
数组有一个“each_with_index”方法，在这种情况下很方便：
my_array.each_with_index do |item, i|
item.do_something if i==0
#common stuff
end
回答by Telemachus
each_with_indexfrom Enumerable(Enumerable is already mixed in with Array, so you can call it on an array without any trouble):
each_with_index来自Enumerable（Enumerable 已经与 Array 混合在一起，因此您可以毫无困难地在数组上调用它）：
irb(main):001:0> nums = (1..10).to_a
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
irb(main):003:0> nums.each_with_index do |num, idx|
irb(main):004:1* if idx == 0
irb(main):005:2> puts "At index #{idx}, the number is #{num}."
irb(main):006:2> end
irb(main):007:1> end
At index 0, the number is 1.
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
回答by steenslag
If you don't need the array afterwards:
如果您之后不需要该数组：
ar = %w(reversed hello world)
puts ar.shift.upcase
ar.each{|item| puts item.reverse}
#=>REVERSED
#=>olleh
#=>dlrow
回答by sarnold
Ruby's Enumerable#injectprovides an argument that can be used for doing something differently on the first iteration of a loop:
RubyEnumerable#inject提供了一个参数，可用于在循环的第一次迭代中做一些不同的事情：
> l=[1,2,3,4]
=> [1, 2, 3, 4]
> l.inject(0) {|sum, elem| sum+elem}
=> 10
The argument is not strictly necessary for common things like sums and products:
对于诸如总和和乘积之类的常见事物，该参数并不是绝对必要的：
> l.inject {|sum, elem| sum+elem}
=> 10
But when you want to do something differenton the first iteration, that argument might be useful to you:
但是当你想在第一次迭代中做一些不同的事情时，这个论点可能对你有用：
> puts fruits.inject("I like to eat: ") {|acc, elem| acc << elem << " "}
I like to eat: apples pears peaches plums oranges
=> nil
回答by android.weasel
Here's a solution that doesn't need to be in an immediately enclosing loop and avoids the redundancy of specifying a status placeholder more than once unless you really need to.
这是一个不需要立即封闭循环的解决方案，并且避免了多次指定状态占位符的冗余，除非您确实需要。
do_this if ($first_time_only ||= [true]).shift
Its scope matches the holder: $first_time_onlywill be globally once; @first_time_onlywill be once for the instance, and first_time_onlywill be once for the current scope.
它的范围与持有者匹配：$first_time_only将是全球性的一次；@first_time_only对于实例first_time_only将是一次，对于当前范围将是一次。
If you want the first several times, etc, you can easily put [1,2,3]if you need to distinguish which of the first iterations you're in, or even something fancy [1, false, 3, 4]if you need something weird.
如果你想要前几次，等等，你可以很容易地把[1,2,3]你是否需要区分你在第一个迭代中的哪个，或者[1, false, 3, 4]如果你需要一些奇怪的东西，甚至是一些花哨的东西。
