find_allのindex版 ref: http://qiita.com/pocari/items/0b4e632ef2a2b6a97ba9

file0.rb

ary = [5, 4, 1, 7, 3, 8]

#案1
p ary.each_with_index.select{|e, i| e >= 5}.map{|e| e[1]} #=>[0, 3, 5]

#案2
p ary.each_with_index.each_with_object([]){|(e, i), acc| acc << i if e >= 5} #=>[0, 3, 5]

file1.txt

#coding: windows-31J
require 'benchmark'

def calc_time(comment)
  val = nil
  t = Benchmark.realtime {
    val = yield
  }
  puts "#{comment} #{t} (msec)"
  val
end

size = 10000000
ary = size.times.map{rand(size)}

calc_time("#案1") {
  ary.each_with_index.select{|e, i| e >= 5}.map{|e| e[1]} #=>[0, 3, 5]
}

calc_time("#案2") {
  ary.each_with_index.each_with_object([]){|(e, i), acc| acc << i if e >= 5} #=>[0, 3, 5]
}

calc_time("#i18nさん") {
  ary.map.with_index { |e,i| e >= 5 ? i : nil }.compact
}

calc_time("#nabetaniさん1") {
  ary.size.times.select{ |x| 5<=ary[x] }  
}

calc_time("#cielavenirさん") {
  ary.each_with_index.lazy.select{|e,i|e>=5}.map(&:last).force
}
calc_time("#cielavenirさん2") {
  ary.flat_map.with_index{|e,i|e>=5 ? i : []}
}
calc_time("#nabetaniさん2") {
  r=[]
  b = ary.size
  loop{
    if b=ary[0,b].rindex{ |x| 5<=x } then r << b else break r.reverse; end
  }
}

#ruby 2.0.0p247 (2013-06-27) [x64-mingw32]
#案1 3.219184 (msec)
#案2 2.044117 (msec)
#i18nさん 1.114063 (msec)
#nabetaniさん1 1.139066 (msec)
#cielavenirさん 7.587434 (msec)
#cielavenirさん2 1.712097 (msec)
#nabetaniさん2 3.168182 (msec)