Question

b1 = Time.now
puts (1..100000).inject(0) { |x, y| x + y }
a1 = Time.now
puts "Time for inject: #{a1 - b1}"

b2 = Time.now
sum = 0
(1..100000).each do |value|
    sum += value
end
puts sum
a2 = Time.now
puts "Time for each: #{a2 - b2}"

The above Ruby code compares two ways of summing up integers. To my surprise, the more elegant inject or reduce approach is outperformed by the other. Why is that the case? Why do people bother using the inefficient inject or reduce? Simply because it’s elegant?

PS: Thanks for all the inspiring answers. My intention was to ask what’s going on behind the scene that results in the differences.

Answer 1

I would go with a little math in this case:

require "benchmark"

N = 5_000_000

Benchmark.bmbm do |bm|
  bm.report "inject 1" do
    (1..N).inject(0) { |x, y| x + y }
  end

  bm.report "inject 2" do
    (1..N).inject(:+)
  end

  bm.report "each" do
    sum = 0
    (1..N).each do |value|
      sum += value
    end
  end

  bm.report "sum of finite arithmetic progression" do
    ((1 + N) * N) / 2
  end
end

And the result is:

% ruby sum.rb
Rehearsal ------------------------------------------------------------------------
inject 1                               0.500000   0.000000   0.500000 (  0.507497)
inject 2                               0.320000   0.000000   0.320000 (  0.322675)
each                                   0.370000   0.000000   0.370000 (  0.380504)
sum of finite arithmetic progression   0.000000   0.000000   0.000000 (  0.000005)
--------------------------------------------------------------- total: 1.190000sec

                                           user     system      total        real
inject 1                               0.500000   0.000000   0.500000 (  0.507697)
inject 2                               0.320000   0.000000   0.320000 (  0.322323)
each                                   0.370000   0.000000   0.370000 (  0.380307)
sum of finite arithmetic progression   0.000000   0.000000   0.000000 (  0.000004)
% 

A better math is always faster 🙂