为什么范围#include？比大于或小于运算符慢得多

Question

我有一个散列数组，键为 Date，值为 Integer。这是模拟它的测试代码。

hashes = 2000.times.map do |i|
  [Date.new(2017) - i.days, rand(100)]
end.to_h

我想获取特定时期的值。一开始我是用Range#include?写的，但是很慢。

Benchmark.measure do
  hashes.select{|k,v| (Date.new(2012,3,3)..Date.new(2012,6,10)).include?(k)}
end

#<Benchmark::Tms:0x007fd16479bed0 @label="", @real=2.9242447479628026, @cstime=0.0, @cutime=0.0, @stime=0.0, @utime=2.920000000000016, @total=2.920000000000016>

使用简单的大于或小于运算符，速度提高了 60 倍。

Benchmark.measure do
  hashes.select{|k,v| k >= Date.new(2012,3,3) && k <= Date.new(2012,6,10)}
end

#<Benchmark::Tms:0x007fd162b61670 @label="", @real=0.05436371313408017, @cstime=0.0, @cutime=0.0, @stime=0.0, @utime=0.05000000000001137, @total=0.05000000000001137>

我以为这两个表达基本一样

为什么差别这么大？

Answer 1

您需要使用 Range#cover? instead of Range#include?, and to calculate the range just once, not once for each element of measure. cover? compares the block variable k with the end-points of the range; include? (for non-numeric objects, such as dates) compares each element in the range with the block variable until it finds a match or concludes there is no match (similar to Array#include?).

此外，您希望考虑 hashes（散列）的每个元素的第一个也是唯一的键，因此如果该散列是 h，则第一个键值对是 h.first，那对的密钥是h.first.first.

require 'date'

Benchmark.measure do
  r = Date.new(2012,3,3)..Date.new(2012,6,10)
  hashes.select{|h| r.cover? h.first.first }
end

就执行速度而言，这应该与您的第二种方法几乎相同。

一个例子

hashes = [{ Date.new(2012,3,1)=>1 }, { Date.new(2012,4,20)=>2 },
          { Date.new(2012,6,10)=>3 }, { Date.new(2012,6,11)=>4 }] 
  #=> [{#<Date: 2012-03-01 ((2455988j,0s,0n),+0s,2299161j)>=>1},
  #    {#<Date: 2012-04-20 ((2456038j,0s,0n),+0s,2299161j)>=>2},
  #    {#<Date: 2012-06-10 ((2456089j,0s,0n),+0s,2299161j)>=>3},
  #    {#<Date: 2012-06-11 ((2456090j,0s,0n),+0s,2299161j)>=>4}] 

r = Date.new(2012,3,3)..Date.new(2012,6,10)
hashes.select{|h| r.cover? h.first.first }
  #=> {#<Date: 2012-04-20 ((2456038j,0s,0n),+0s,2299161j)>=>2,
  #    #<Date: 2012-06-10 ((2456089j,0s,0n),+0s,2299161j)>=>3}

为什么范围#include？比大于或小于运算符慢得多

Why Range#include? is much slower than greater or less than operator

ruby

activesupport