Ruby - 将数组映射到哈希图
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Ruby - mapping an array to hashmap
提问by Ji Mun
I have an array, and a function that returns a value given a value. Ultimately I want to create a hashmap that has the values of the array as key value, and the result of f(key_value) as the value. Is there a clean, simple way, like similar to each/map of Array, of doing this using block?
我有一个数组和一个返回给定值的值的函数。最终我想创建一个哈希图，它以数组的值作为键值，并将 f(key_value) 的结果作为值。是否有一种干净，简单的方法，类似于数组的 each/map，使用块来执行此操作？
So something that is equivalent to
所以相当于
hsh = {}
[1,2,3,4].each do |x|
hsh[x] = f(x)
end
but looks more similar to this, in that it's simple and one line?
但看起来更像这个，因为它很简单而且只有一行？
results = array.map { | x | f(x) }
采纳答案by Zach Kemp
You could also define the function as the hash's default value:
您还可以将该函数定义为散列的默认值：
hash = Hash.new {|hash, key| hash[key] = f(key) }
Then when you lookup a value, the hash will calculate and store it on the fly.
然后，当您查找一个值时，散列将即时计算并存储它。
hash[10]
hash.inspect #=> { 10 => whatever_the_result_is }
回答by Knotty66
Note that since Ruby 2.1.0 you can also use Array#to_h, like this:
请注意，从 Ruby 2.1.0 开始，您还可以使用Array#to_h，如下所示：
[1,2,3,4].map{ |x| [x, f(x)] }.to_h
回答by Sergio Tulentsev
You need each_with_object.
你需要each_with_object.
def f x
x * 2
end
t = [1, 2, 3, 4].each_with_object({}) do |x, memo|
memo[x] = f(x)
end
t # => {1=>2, 2=>4, 3=>6, 4=>8}
Another one:
另一个：
t2 = [1, 2, 3, 4].map{|x| [x, f(x)]}
Hash[t2] # => {1=>2, 2=>4, 3=>6, 4=>8}
回答by Matt Huggins
回答by Timitry
Ruby 2.6.0 enables passing a block to the to_h-method. This enables an even shorter syntax for creating a hash from an array:
Ruby 2.6.0 允许将块传递给to_h-method。这为从数组创建哈希启用了更短的语法：
[1, 2, 3, 4].to_h { |x| [x, f(x)] }
回答by tokland
回答by Tombart
You're looking for reduce()|inject()method:
您正在寻找reduce()|inject()方法：
elem = [1,2,3,4]
h = elem.reduce({}) do |res, x|
res[x] = x**2
res
end
puts h
The argument passed to reduce({})is the initial value of an intermediate object that is passed to the block as resvariable. In each iteration we're adding new pair key: valueto the resHash and returing the Hash to be used in next iteration.
传递给的参数reduce({})是作为res变量传递给块的中间对象的初始值。在每次迭代中，我们都将新对添加key: value到resHash 并返回要在下一次迭代中使用的 Hash。
The method above precomputes a very practical hash of squared values:
上面的方法预先计算了一个非常实用的平方值散列：
{1=>1, 2=>4, 3=>9, 4=>16}
