Ruby select latest duplicated values from an array of hash - ruby

Question

Let say I have this kind of array
a = [
{key: "cat", value: 1},
{key: "dog", value: 2},
{key: "mouse", value: 5},
{key: "rat", value: 3},
{key: "cat", value: 5},
{key: "rat", value: 2},
{key: "cat", value: 1},
{key: "cat", value: 1}
]
Let say I have this array, and want to get only the latest value found for "cat".
I know how to select all of them
like
a.select do |e|
e[:key] == "cat"
end
But I'm looking for a way to just get a selection of the last 3
desired result would be
[
{key: "cat", value: 5},
{key: "cat", value: 1},
{key: "cat", value: 1}
]
thanks!

Answer 1

In a comment on the question #Stefan suggested:
a.select { |e| e[:key] == "cat" }.last(3)
Provided a is not too large that is likely what you should use. However, if a is large, and especially if it contains many elements (hashes) h for which h[:key] #=> "cat", it likely would be more efficient to iterate backwards from the end of the array and terminate ("short-circuit") as soon as three elements h have been found for which h[:key] #=> "cat". This also avoids the construction of a potentially-large temporary array (a.select { |e| e[:key] == "cat" }).
One way to do that is as follows.
a.reverse_each.with_object([]) do |h,arr|
arr.insert(0,h) if h[:key] == "cat"
break arr if arr.size == 3
end
#=> [{:key=>"cat", :value=>5},
# {:key=>"cat", :value=>1},
# {:key=>"cat", :value=>1}]
See Array#reverse_each, Enumerator#with_object and Array#insert. Note that because reverse_each and with_object both return enumerators, chaining them produces an enumerator as well:
a.reverse_each.with_object([])
#=> #<Enumerator: #<Enumerator: [{:key=>"cat", :value=>1},
# ...
# {:key=>"cat", :value=>1}]:reverse_each>:with_object([])>
It might be ever-so-slightly faster to replace the block calculation with
arr << h if h[:key] == "cat"
break arr.reverse if arr.size == 3
If a contains fewer elements h for which h[:key] #=> "cat" an array arr will be returned for which arr.size < 3. It therefore is necessary to confirm that the array returned contains three elements.
This check must also be performed when #Stefan's suggested code is used, as (for example)
a.select { |e| e[:key] == "cat" }.last(99)
#=> [{:key=>"cat", :value=>1},
# {:key=>"cat", :value=>5},
# {:key=>"cat", :value=>1},
# {:key=>"cat", :value=>1}]