Change: add another slicing method, and add a note about assuming all…

… keys being in the Hash.

README.md

@@ -1038,24 +1038,27 @@ 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 `Hash#slice#values`
-[code](code/hash/values_at-vs-slice-values.rb)
+##### `Hash#values_at` vs `Hash#slice#values` vs `Array#map { Hash#[] }`
+[code](code/hash/values_at-vs-map-vs-slice-values.rb)
 
-To select hash values by keys.
+To select hash values by keys, when all the keys exist in the hash.
 
 ```
-$ ruby -v code/hash/values_at-vs-slice-values.rb
-ruby 2.6.1p33 (2019-01-30 revision 66950) [x86_64-darwin18]
+$ ruby -v code/hash/values_at-vs-map-vs-slice-values.rb
+ruby 2.6.2p47 (2019-03-13 revision 67232) [x86_64-darwin18]
 Warming up --------------------------------------
-   Hash#values_at      284.628k i/100ms
-   Hash#slice#values   230.279k i/100ms
+   Hash#values_at      298.254k i/100ms
+   Array#map           250.633k i/100ms
+   Hash#slice#values   243.267k i/100ms
 Calculating -------------------------------------
-   Hash#values_at         7.179M (± 7.4%) i/s -     35.863M in   5.034179s
-   Hash#slice#values      4.458M (± 4.1%) i/s -     22.337M in   5.019922s
+   Hash#values_at         7.540M (± 1.5%) i/s -     37.878M in   5.025027s
+   Array#map              5.119M (± 2.3%) i/s -     25.815M in   5.045566s
+   Hash#slice#values      4.817M (± 1.7%) i/s -     24.083M in   5.001133s
 
 Comparison:
-   Hash#values_at   :  7178610.6 i/s
-   Hash#slice#values:  4458017.0 i/s - 1.61x  slower
+   Hash#values_at   :  7539658.6 i/s
+   Array#map        :  5119405.7 i/s - 1.47x  slower
+   Hash#slice#values:  4817081.6 i/s - 1.57x  slower
 ```
 
 

code/hash/values_at-vs-map-vs-slice-values.rb

@@ -0,0 +1,30 @@
+require 'benchmark/ips'
+
+HASH = {
+  a: 'foo',
+  b: 'bar',
+  c: 'baz',
+  d: 'qux'
+}.freeze
+
+# Only keys that exist in the hash.
+KEYS = %i[a c].freeze
+
+def fast
+  HASH.values_at(*KEYS)
+end
+
+def slow
+  KEYS.map { |key| HASH[key] }
+end
+
+def slowest
+  HASH.slice(*KEYS).values
+end
+
+Benchmark.ips do |x|
+  x.report('Hash#values_at       ') { fast }
+  x.report('Array#map { Hash#[] }') { slow }
+  x.report('Hash#slice#values    ') { slowest }
+  x.compare!
+end