Add Hash#values_at vs Array#map { Hash#[] } comparison

README.md

@@ -1038,6 +1038,26 @@ Array#each_w/_object: 1352851.8 i/s - 1.88x slower
 Hash#select-include : 760944.2 i/s - 3.34x slower
 ```
 
+##### `Hash#values_at` vs `Array#map { Hash#[] }`
+[code](code/hash/values_at-vs-map.rb)
+
+To select hash values by keys, when some of the keys may not exist in the hash,
+and you care about the default values. 
+
+```
+$ ruby -v code/hash/values_at-vs-map.rb
+ruby 3.2.1 (2023-02-08 revision 31819e82c8) [x86_64-darwin22]
+Warming up --------------------------------------
+Hash#values_at 503.783k i/100ms
+Array#map { Hash#[] } 279.576k i/100ms
+Calculating -------------------------------------
+Hash#values_at 4.901M (± 4.1%) i/s - 24.685M in 5.046090s
+Array#map { Hash#[] } 2.801M (± 3.1%) i/s - 14.258M in 5.095692s
+
+Comparison:
+Hash#values_at : 4900567.9 i/s
+Array#map { Hash#[] }: 2800981.4 i/s - 1.75x slower
+```
 
 ### Proc & Block
 

code/hash/values_at-vs-map.rb

@@ -0,0 +1,25 @@
+require 'benchmark/ips'
+
+HASH = {
+ a: 'foo',
+ b: 'bar',
+ c: 'baz',
+ d: 'qux'
+}.freeze
+
+# Some of the keys may not exist in the hash; we want to keep the default values.
+KEYS = %i[a c e f].freeze
+
+def fast
+ HASH.values_at(*KEYS)
+end
+
+def slow
+ KEYS.map { |key| HASH[key] }
+end
+
+Benchmark.ips do |x|
+ x.report('Hash#values_at ') { fast }
+ x.report('Array#map { Hash#[] }') { slow }
+ x.compare!
+end