Ruby：如何从嵌套哈希中删除nil / empty值，为什么我的尝试不这样做呢？

Question

I'm trying to write a method that removes all keys in a nested hash that point to nil recursively.

For example:

{:a=>nil, :b=>"b", :c=>nil, :d=>{:dd=>"dd", :ee=>nil, :ff=>"ff"}, :e=>{:gg=>nil, :hh=>nil}}

becomes:

{:b=>"b", :d=>{:dd=>"dd", :ff=>"ff"}}

I'm not having much luck though.

My most recent attempt looks like:

def deep_compact(hash)
  hash.reject do |key, value|
    deep_compact(value) if value.class == Hash
    next true if value.nil? || value.empty?
  end
end

Here I want to iterate over each key value pair in the hash. If the value is a hash, I want to do the same for that hash. I want to reject the pair if the value is nil or empty. Otherwise, I want to keep it.

The result isn't what I want:

#=> {:b=>"b", :d=>{:dd=>"dd", :ee=>nil, :ff=>"ff"}, :e=>{:gg=>nil, :hh=>nil}}

I have also tried:

def deep_compact(hash)
  hash.compact.transform_values do |value|
    deep_compact(value) if value.class == Hash
    value
  end
end

Again, I get the same result:

#=> {:b=>"b", :d=>{:dd=>"dd", :ee=>nil, :ff=>"ff"}, :e=>{:gg=>nil, :hh=>nil}}

This leaves me to believe that either I've missed something or my understanding of recursion is wrong.

Are any of my attempts close? What do I need to do to ensure I get the result I want: {:b=>"b", :d=>{:dd=>"dd", :ff=>"ff"}} ?

Answer 1

The trick would be to recursively compact nested hashes and then to eliminate empty values.

compact = ->(hash) { 
  hash.is_a?(Hash) ?
    hash.map { |k, v| [k, compact.(v)] }.
         to_h.
         delete_if { |_, v| v.nil? || v.respond_to?(:empty?) && v.empty? } :
    hash 
}
compact.(input)
#⇒ {:b=>"b", :d=>{:dd=>"dd", :ff=>"ff"}}

Answer 2

I discovered that by placing my recursive function call at the end of the block got me most of the way there. (Is this 'tail-end' recursion?)

I also call reject on the hash returned by transform_values to removed any empty pairs.

This achieves what I wanted:

def deep_compact(hash)
  hash.compact.transform_values do |value|
    next value unless value.class == Hash
    deep_compact(value)
  end.reject { |_k, v| v.empty? }
end

> h
=> {:a=>nil, :b=>"b", :c=>nil, :d=>{:dd=>"dd", :ee=>nil, :ff=>"ff"}, :e=>{:gg=>nil, :hh=>nil}}
> deep_compact h
=> {:b=>"b", :d=>{:dd=>"dd", :ff=>"ff"}}

Answer 3

Other option using Hash#reject! , it changes the original Hash:

def deep_compact(h)
  h.each { |_, v| deep_compact(v) if v.is_a? Hash }.reject! { |_, v| v.nil? || v.empty? }
end

deep_compact(h)
#=> {:b=>"b", :d=>{:dd=>"dd", :ff=>"ff"}}