blocks|key|727379|text|您可以这样做：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|727380|my_array.each_with_index+do+%7Citem,+index%7C
++++if+index+==+0
++++++++#+do+something+with+the+first+item
++++end
++++#+common+stuff
end|code-block|syntax|javascript|727381|在ideone上试试吧。|offset|length|727382|entityMap|0|LINK|mutability|MUTABLE|url|http://ideone.com/JXGph^0|0|0|1|6|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@$I|V|J|W|1|X]]|A|$]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|L|$M|$5|N|O|P|A|$Q|R]]]]

You can do this:

<pre><code>my_array.each_with_index do |item, index|
 if index == 0
 # do something with the first item
 end
 # common stuff
end
</code></pre>

Try it on <a href="http://ideone.com/JXGph">ideone</a>.

blocks|key|511033|text|来自Enumerable的枚举(Enumerable已经和数组混在一起了，所以你可以毫不费力地在数组上调用它)：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|511034|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+%7Cnum,+idx%7C
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]|code-block|syntax|javascript|511035|entityMap|0|LINK|mutability|MUTABLE|url|http://www.ruby-doc.org/core-1.9.3/Enumerable.html#method-i-each_with_index^0|2|A|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

<code>each_with_index</code> from <a href="http://www.ruby-doc.org/core-1.9.3/Enumerable.html#method-i-each_with_index" rel="nofollow">Enumerable</a> (Enumerable is already mixed in with Array, so you can call it on an array without any trouble):

<pre><code>irb(main):001:0&gt; nums = (1..10).to_a
=&gt; [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
irb(main):003:0&gt; nums.each_with_index do |num, idx|
irb(main):004:1* if idx == 0
irb(main):005:2&gt; puts "At index #{idx}, the number is #{num}."
irb(main):006:2&gt; end
irb(main):007:1&gt; end
At index 0, the number is 1.
=&gt; [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
</code></pre>

blocks|key|511087|text|数组有一个"each_with_index“方法，这个方法在这种情况下很方便：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|511088|my_array.each_with_index+do+%7Citem,+i%7C
++item.do_something+if+i==0
++#common+stuff
end|code-block|syntax|javascript|511089|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Arrays have an "each_with_index" method which is handy for this situation:

<pre><code>my_array.each_with_index do |item, i|
 item.do_something if i==0
 #common stuff
end
</code></pre>

blocks|key|727560|text|正如其他人所描述的那样，使用each_with_index可以很好地工作，但考虑到多样性的原因，这里是另一种方法。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|727561|如果您只想对第一个元素执行特定操作，而对包括第一个元素在内的所有元素执行通用操作，则可以这样做：|727562|#+do+something+with+my_array[0]+or+my_array.first
my_array.each+do+%7Ce%7C+
++#+do+the+same+general+thing+to+all+elements+
end|code-block|syntax|javascript|727563|但是如果你不想对第一个元素做一般的事情，你可以这样做：|727564|#+do+something+with+my_array[0]+or+my_array.first
my_array.drop(1).each+do+%7Ce%7C+
++#+do+the+same+general+thing+to+all+elements+except+the+first+
end|727565|entityMap^0|E|F|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|V|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|W|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|X|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|Y|8|@]|D|@]|E|$K|L]]|$1|Q|3|-4|5|6|7|Z|8|@]|D|@]|E|$]]]|R|$]]

Using <code>each_with_index</code>, as others have described, would work fine, but for the sake of variety here is another approach.

If you want to do something specific for the first element only and something general for all elements including the first, you could do:

<pre><code># do something with my_array[0] or my_array.first
my_array.each do |e| 
 # do the same general thing to all elements 
end
</code></pre>

But if you want to not do the general thing with the first element you could do:

<pre><code># 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
</code></pre>

blocks|key|727599|text|最合适的选择取决于具体情况。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|727600|另一种选择(如果您知道数组不为空)：|727601|#+treat+the+first+element+(my_array.first)
my_array.each+do+%7C+item+%7C
+++#+do+the+common_stuff
end|code-block|syntax|javascript|727602|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

What fits best is depending on the situation.

Another option (if you know your array is not empty):

<pre><code># treat the first element (my_array.first)
my_array.each do | item |
 # do the common_stuff
end
</code></pre>

blocks|key|511178|text|如果以后不需要该数组：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|511179|ar+=+%25w(reversed+hello+world)

puts+ar.shift.upcase
ar.each{%7Citem%7C+puts+item.reverse}

#=>REVERSED
#=>olleh
#=>dlrow|code-block|syntax|javascript|511180|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

If you don't need the array afterwards:

<pre><code>ar = %w(reversed hello world)

puts ar.shift.upcase
ar.each{|item| puts item.reverse}

#=&gt;REVERSED
#=&gt;olleh
#=&gt;dlrow
</code></pre>

blocks|key|511226|text|这里有一个解决方案，它不需要在立即封闭的循环中，并且避免了多次指定状态占位符的冗余，除非您真的需要这样做。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|511227|do_this+if+($first_time_only+%7C%7C=+[true]).shift|code-block|syntax|javascript|511228|它的作用域与持有者匹配：$first_time_only是全局的，@first_time_only是实例的一次，first_time_only是当前作用域的一次。|offset|length|style|CODE|511229|如果你想要第一次迭代，等等，如果你需要区分你在第一次迭代中的哪一个，你可以很容易地放入[1,2,3]，或者如果你需要一些奇怪的东西，甚至可以放一些花哨的[1,+false,+3,+4]。|511230|entityMap^0|0|0|C|G|X|G|1K|F|0|17|7|24|G|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@$I|T|J|U|K|L]|$I|V|J|W|K|L]|$I|X|J|Y|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|Z|8|@$I|10|J|11|K|L]|$I|12|J|13|K|L]]|9|@]|A|$]]|$1|O|3|-4|5|6|7|14|8|@]|9|@]|A|$]]]|P|$]]

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.

<pre><code>do_this if ($first_time_only ||= [true]).shift
</code></pre>

Its scope matches the holder: <code>$first_time_only</code> will be globally once; <code>@first_time_only</code> will be once for the instance, and <code>first_time_only</code> will be once for the current scope.

If you want the first several times, etc, you can easily put <code>[1,2,3]</code> if you need to distinguish which of the first iterations you're in, or even something fancy <code>[1, false, 3, 4]</code> if you need something weird.

I always use a counter to check for the first item (<code>i==0</code>) in a loop:

<pre><code>i = 0
my_array.each do |item|
 if i==0
 # do something with the first item
 end
 # common stuff
 i += 1
end
</code></pre>

Is there a more elegant way to do this (perhaps a method)?

How to act differently on the first iteration in a Ruby loop?

我总是使用计数器来检查循环中的第一项(i==0)：i = 0my_array.each do |item|  if i==0    # do something with the first item  end  # common stuff  i += 1end有没有一种更优雅的方法(也许是一种方法)呢？

问如何在Ruby循环的第一次迭代中采取不同的行为？
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