在训练营练习中，我们得到了这个数组：
library = [
{
"title" => "Hitchhiker's Guide",
"Author" => "Douglas Adams",
"categories" => [ "comedy", "fiction", "british"]
},
{
"title" => "Pride and Prejudice",
"Author" => "Jane Austen",
"categories" => [ "morality", "fiction", "society", "british"]
},
{
"title" => "Search Inside Yourself",
"Author" => "Chade-Meng Tan",
"categories" => [ "self improvement", "non-fiction", "mindfulness", "business"]
},
{
"title" => "Benjamin Franklin: An American Life",
"Author" => "Walter Isaacson",
"categories" => [ "non-fiction", "history", "founding fathers"]
},
{
"title" => "Glengarry Glen Ross",
"Author" => "David Mamet",
"categories" => [ "play", "fiction", "drama"]
}
]
我们需要创建一个新的哈希值，将类别作为元素，将标题作为值。
能够弄清楚如何制作category_hash，但我无法弄清楚如何将标题附加到每个类别。这是我的代码：
category_hash = {}
sub_category = []
library.each do |book|
book["categories"].each do |category|
sub_category << category
end
sub_category.each do |index|
category_hash[index] = "I'm not sure how to append the titles here"
end
end
p category_hash
有人可以帮我弄清楚这一步吗？
您可以使用Enumerable#each_with_object：
library.each_with_object({}) do |hash, result|
result[hash['categories']] = hash['title']
end
# {
# ["comedy", "fiction", "british"]=>"Hitchhiker's Guide",
# ["morality", "fiction", "society", "british"]=>"Pride and Prejudice",
# ["self improvement", "non-fiction", "mindfulness", "business"]=>"Search Inside Yourself",
# ["non-fiction", "history", "founding fathers"]=>"Benjamin Franklin: An American Life",
# ["play", "fiction", "drama"]=>"Glengarry Glen Ross"
# }
这是另一种方法，它比@AndreyDeineko的答案效率低。
library.map{|h| h.values_at("categories", "title")}.to_h
结果：
{
["comedy", "fiction", "british"]=>"Hitchhiker's Guide",
["morality", "fiction", "society", "british"]=>"Pride and Prejudice",
["self improvement", "non-fiction", "mindfulness", "business"]=>"Search Inside Yourself",
["non-fiction", "history", "founding fathers"]=>"Benjamin Franklin: An American Life",
["play", "fiction", "drama"]=>"Glengarry Glen Ross"
}
如果您希望它按"category"属性分布(当然，这将取决于数组中哈希项的顺序;我假设你的意思是后来的优先于较早的(：
library.each_with_object({}) do
|h, result| h["categories"].each_with_object(result) do
|k, result| result[k] = h["title"]
end
end
结果：
{
"comedy"=>"Hitchhiker's Guide",
"fiction"=>"Glengarry Glen Ross",
"british"=>"Pride and Prejudice",
"morality"=>"Pride and Prejudice",
"society"=>"Pride and Prejudice",
"self improvement"=>"Search Inside Yourself",
"non-fiction"=>"Benjamin Franklin: An American Life",
"mindfulness"=>"Search Inside Yourself",
"business"=>"Search Inside Yourself",
"history"=>"Benjamin Franklin: An American Life",
"founding fathers"=>"Benjamin Franklin: An American Life",
"play"=>"Glengarry Glen Ross",
"drama"=>"Glengarry Glen Ross"
}
如果您希望一个类别中每本书的列表，以及一本书出现在多个类别中，则可以在初始化 Hash 时附加一个default_proc，以将所有未知键设置为包含数组作为值。然后，您只需迭代图书馆中的每本书，并将其标题添加到其所在每个类别的列表中：
# Pass the category hash a default proc, which will be called
# whenever a key it doesn't have is accessed. So, when you
# encounter a brand new category, it'll already be set up with
# an array
category_hash = Hash.new { |hash, key| hash[key] = [] }
library.each do |book|
book['categories'].each do |category|
# since we passed the default, no worrying, an array will be
# there and we can just add our value into it
category_hash[category] << book['title']
end
end
puts category_hash
结果：
{
"comedy"=>["Hitchhiker's Guide"],
"fiction"=>["Hitchhiker's Guide", "Pride and Prejudice", "Glengarry Glen Ross"],
"british"=>["Hitchhiker's Guide", "Pride and Prejudice"],
"morality"=>["Pride and Prejudice"],
"society"=>["Pride and Prejudice"],
"self improvement"=>["Search Inside Yourself"],
"non-fiction"=>["Search Inside Yourself", "Benjamin Franklin: An American Life"],
"mindfulness"=>["Search Inside Yourself"],
"business"=>["Search Inside Yourself"],
"history"=>["Benjamin Franklin: An American Life"],
"founding fathers"=>["Benjamin Franklin: An American Life"],
"play"=>["Glengarry Glen Ross"],
"drama"=>["Glengarry Glen Ross"]
}
还有each_with_object，因此您可以将其修改为
result = library.each.with_object(Hash.new { |hash, key| hash[key] = [] }) do |book, category_hash|
book['categories'].each do |category|
# since we passed the default, no worrying, an array will be
# there and we can just add our value into it
category_hash[category] << book['title']
end
end
puts result
这将返回category_hash，并保存最后一行代码：
puts(library.each.with_object(Hash.new { |hash, key| hash[key] = [] }) do |book, category_hash|
book['categories'].each do |category|
# since we passed the default, no worrying, an array will be
# there and we can just add our value into it
category_hash[category] << book['title']
end
end)
这是其他答案的轻微变体。
library = [
{title: "Hitch Guide", by: "Doug", cats: ["comedy", "fiction", "british"]},
{title: "P and P", by: "Jane", cats: ["morality", "fiction", "british"]},
{title: "Searchin'", by: "Chad", cats: ["non-fiction", "morality", "gps"]},
{title: "Crazy Ben", by: "Walt", cats: ["non-fiction", "comedy", "dads"]}
]
# => [{:title=>"Hitch Guide", :by=>"Doug", :cats=>["comedy", "fiction", "british"]},
# {:title=>"P and P", :by=>"Jane", :cats=>["morality", "fiction", "british"]},
# {:title=>"Searchin'", :by=>"Chad", :cats=>["non-fiction", "morality", "gps"]},
# {:title=>"Crazy Ben", :by=>"Walt", :cats=>["non-fiction", "comedy", "dads"]}]
library.each_with_object({}) do |g,h|
title = g[:title]
g[:cats].each { |c| (h[c] ||= []) << title }
end
#=> {"comedy"=>["Hitch Guide", "Crazy Ben"],
# "fiction"=>["Hitch Guide", "P and P"],
# "british"=>["Hitch Guide", "P and P"],
# "morality"=>["P and P", "Searchin'"],
# "non-fiction"=>["Searchin'", "Crazy Ben"],
# "gps"=>["Searchin'"],
# "dads"=>["Crazy Ben"]}
这就是我通常的做法，我认为这是最简单的方法，尽管上面看到的each_with_object技巧也很巧妙。
new_hash = {}
library.each do |hash|
hash["categories"].each do |category|
new_hash[category] ||= []
new_hash[category] << hash["title"]
end
end