我可以在Ruby中创建使用默认值的数组？ [英] Can I create an array in Ruby with default values?

问题描述

Perl是pretty关于默认值漂亮的：

Perl is pretty nice about default values:

: jmglov@laurana; perl -e '@foo; printf "%d\n", $foo[123]'
0
: jmglov@laurana; perl -e '%foo; printf "%d\n", $foo{bar}'
0

红宝石可以做同样的，至少对散列：

Ruby can do the same, at least for hashes:

>> foo = Hash.new(0)
=> {}
>> foo[:bar]
=> 0

不过貌似同样不数组工作：

But the same seemingly does not work for arrays:

>> foo = Array.new(0)
=> []
>> foo[123]
=> nil
>> foo[124] = 0
=> 0
>> foo[456] = 0
=> 0
>> foo[455,456]
=> [nil, 0]

是否有可能提供了数组的默认值，所以当他们自动延长，他们充满了0，而不是零？

Is it possible to supply a default value for arrays, so when they are auto-extended, they're filled with 0 instead of nil?

当然，我可以解决此问题，但在成本前pressiveness：

Of course I can work around this, but at a cost to expressiveness:

>> foo[457,458] = 890, 321
=> [890, 321]
>> foo[456] += 789
NoMethodError: You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.+
>> foo.inject(0) {|sum, i| sum += (i || 0) }
=> 1211
>> foo.inject(:+)
NoMethodError: You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.+

更新1：我的一个同事指出，我可以使用 #compact 来解决 #inject 的问题， #to_i 来解决标准的元素在指数的问题：

Update 1: One of my colleagues pointed out that I can use #compact to solve the #inject issue, and #to_i to solve the standard element-at-index issue:

>> foo.include? nil
=> true
>> foo.compact.inject(:+)
=> 1211
>> foo[456,457]
=> [0, 890, 321]
>> foo[455..457]
=> [nil, 0, 890]
>> foo[455..457].map(&:to_i)
=> [0, 0, 890]

更新2：感谢安德鲁格林为解决 + = 问题：

>> foo = []
=> []
>> def foo.[](i)
>>   fetch(i) {0}
>> end
=> nil
>> foo[4]
=> 0
>> foo
=> []
>> foo[4] += 123
=> 123
>> foo
=> [nil, nil, nil, nil, 123]

更新3：这是开始看起来像捶一个痣

Update 3: this is starting to look like whack-a-mole!

>> foo
=> [nil, nil, nil, nil, 123]
>> foo[-2..-1]
TypeError: can't convert Range into Integer

但是，我们可以处理的：

But we can deal with that:

>> def foo.[](index)
>>   if index.is_a? Range
>>     index.map {|i| self[i] }
>>   else
?>     fetch(index) { 0 }  # default to 0 if no element at index; will not cause auto-extension of array
>>   end
>> end
=> nil
>> foo
=> [nil, nil, nil, nil, 123]
>> foo[-2..-1]
=> [nil, 123]

我现在不得不承认（不好意思地）我会继承阵列来避免弄乱我的code：

I now have to admit (sheepishly) that I'll subclass Array to avoid cluttering my code:

class MyClass
  class ArrayWithDefault < Array
    def [](index)
      if index.is_a? Range
        index.map {|i| self[i] }
      else
        fetch(index) { 0 }  # default to 0 if no element at index; will not cause auto-extension of array
      end
    end
  end
end

感谢所有的创造性的解决方案。 TIMTOWTDI哉！

Thanks for all the creative solutions. TIMTOWTDI indeed!

推荐答案

由于红宝石返回无为不存在的元素（相对指数外-bounds类型的错误），你可以只使用一个或

Given that Ruby returns nil for a non-existing element (as opposed to index-out-of-bounds type error), you could just use an "or":

a = [1,2,3]
puts a[5]  # => nil
puts a[5] || "a default"  # => a default

您可以采取猴子打补丁的方法，但你可能不希望这样做的任何事情比1文件脚本较大的：

You could take the monkey patch approach, but you probably would not want to do this in anything larger than a 1-file script:

a = [1,2,3]
def a.[](index)
  self.at(index) ? self.at(index) : "a default"
end
puts a[5]   # => "a default"

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