From: Colin Bartlett <colinb2r@...>
Date: 2011-01-25T06:43:41+09:00
Subject: Re: Enumerable.inject_with_index?

I'm curious as to when and why people use each_with_index and/or
inject, etc, instead of a somewhat less elegant, and admittedly
possibly less flexible, more direct approach. (Which usually(?) takes
about the same number of characters for the code.)

ary = [3, 4, 5]
rrr = ary.each_with_index.inject(0) { |sum, (v, index)| sum + v * index }
p rrr #=> 14
index = -1; rrr = ary.inject(0) { |sum, v| index += 1; sum + v * index }
p rrr #=> 14
rrr = 0; ary.each_with_index { |v, index| rrr += v * index }; rrr
p rrr #=> 14
index = -1; rrr = 0; ary.each { |v| index += 1; rrr += v * index }; rrr
p rrr #=> 14

require "benchmark"
kt = 500_000
Benchmark.bmbm() do |bm|
  bm.report( "each_with_index.inject(0)" ) { kt.times {
   rrr = ary.each_with_index.inject(0) { |sum, (v, index)| sum + v * index }
  } }
  bm.report( "inject(0)"                 ) { kt.times {
   index = -1; rrr = ary.inject(0) { |sum, v| index += 1; sum + v * index }
  } }
  bm.report( "each_with_index"           ) { kt.times {
   rrr = 0; ary.each_with_index { |v, index| rrr += v * index }
  } }
  bm.report( "each"                      ) { kt.times {
   index = -1; rrr = 0; ary.each { |v| index += 1; rrr += v * index }
  } }
end

#=>
Rehearsal -------------------------------------------------------------
each_with_index.inject(0)   2.410000   0.000000   2.410000 (  2.404242)
inject(0)                   1.000000   0.000000   1.000000 (  1.000677)
each_with_index             0.840000   0.010000   0.850000 (  0.846287)
each                        0.640000   0.000000   0.640000 (  0.640142)
---------------------------------------------------- total: 4.900000sec

                                user     system      total        real
each_with_index.inject(0)   2.380000   0.020000   2.400000 (  2.392590)
inject(0)                   1.000000   0.000000   1.000000 (  0.999785)
each_with_index             0.850000   0.000000   0.850000 (  0.846178)
each                        0.640000   0.000000   0.640000 (  0.639366)

(My apologies if the benchmarks are ragged and unaligned.)