从哈希数组中获取基于字符串的唯一元素

Question

I have an array of hashes:

[{:a => "63-aaa",:b =>1}, {:a => "90-bbb",:b => 89}, {:a => "63-aaa", :b => 1},{:a => "45-hhh", :b => 44}, {:a => "32-kkk", :b => 67}, {: => "90-bbb", :b => 89}]

a key has value 63-aaa , 90-bbb , 45-hhh and 32-kkk . I want to get the unique elements of the array on the basis of ids of values of a like:

[{:a => "63-aaa",:b =>1}, {:a => "90-bbb",:b => 89}, {:a => "45-hhh", :b => 44}, {:a => "32-kkk", :b => 67}]

Answer 1

在这种情况下，您可以将块传递给Array#uniq ：

arr.uniq { |hash| hash[:a] }

Answer 2

You don't appear to care which one of each set of duplicates is kept. Array#uniq , with a block, is purpose-built for that. It will keep the one with the smallest index. If you want the one with the largest index, just apply Array#reverse before and after uniq .

If, however, you wanted to use other information in each hash to determine which one to keep, you could do that by using the form of Hash#update (aka merge! ) that takes a block:

arr = [{:a=>"63-aaa", :b=> 1}, {:a=>"90-bbb", :b=>89}, {:a=>"63-aaa", :b=>22},
       {:a=>"45-hhh", :b=>44}, {:a=>"32-kkk", :b=>67}, {:a=>"90-bbb", :b=>14}]

arr.each_with_object({}) do |g,h|
  h.update({ g[:a]=>g }) { |k,oh,nh| <code to return oh or nh> }
end.values

Suppose, for example, you want to keep the hash h for which h[:b] is largest:

arr.each_with_object({}) do |g,h|
  h.update({ g[:a]=>g }) { |k,oh,nh| nh[:b] > oh[:b] ? nh : oh }
end.values
  #=> [{:a=>"63-aaa", :b=>22}, {:a=>"90-bbb", :b=>89},
  #    {:a=>"45-hhh", :b=>44}, {:a=>"32-kkk", :b=>67}]

I have created an empty hash (block variable h ), and then for each hash g in arr , update h with the hash f = { g[:a]=>g } . If both h and f have the key g[:a] , the block

{ |k,oh,nh| (nh[:b] > oh[:b]) ? nh : oh  }

is called upon to determine the value of the key g[:a] in h (that is, which of the two hashes to keep). The block variables are:

k  = g[:a]
oh = h[g[:a]]
nh = g

(Note that k is not used in the block, so we might write the block variables as |_,oh,nh| .)

each_with_object returns

h = {"63-aaa"=>{:a=>"63-aaa", :b=>22}, "90-bbb"=>{:a=>"90-bbb", :b=>89},
     "45-hhh"=>{:a=>"45-hhh", :b=>44}, "32-kkk"=>{:a=>"32-kkk", :b=>67}}

so we merely have to extract the values:

h.values
  #=> [{:a=>"63-aaa", :b=>22}, {:a=>"90-bbb", :b=>89},
  #    {:a=>"45-hhh", :b=>44}, {:a=>"32-kkk", :b=>67}]

Another way is to first execute:

arr.group_by { |h| h[:a] }.values
  #=> [[{:a=>"63-aaa", :b=> 1}, {:a=>"63-aaa", :b=>22}],
  #    [{:a=>"90-bbb", :b=>89}, {:a=>"90-bbb", :b=>14}],
  #    [{:a=>"45-hhh", :b=>44}],
  #    [{:a=>"32-kkk", :b=>67}]]

and then use whatever criteria you like to select one hash from each of the four arrays.