From: Mauricio Fernandez <mfp@...>
Date: 2006-04-13T23:32:44+09:00
Subject: Re: map_if, collect_if ???

On Thu, Apr 13, 2006 at 10:53:24PM +0900, Robert Dober wrote:
> On 4/13/06, Robert Dober <robert.dober@gmail.com> wrote:
> module Enumerable
>     def map_if
>         map{ |x| (r = yield(x)) ? r : nil}.compact
>     end
> end
> 
> that one might do as a comprimise between speed, elegance and sanity?

It's quite slow (two blocks in use...).

If you care a bit about performance (but not enough to use C),

  def map_if(&b)
    a = map(&b)
    a.delete(false) # to get the same semantics as select{|e| e}
    a.compact!
    a
  end

requires half as much mem in the worst case as, and runs ~60% faster than
map(&b).select{|e| e}:

RUBY_VERSION                                       # => "1.8.4"
module Enumerable
  def map_if(&b)
    a = map(&b)
    a.delete(false) # to get the same semantics as select{|e| e}
    a.compact!
    a
  end

  def map_if2(&b)
    map(&b).select{|e| e}
  end

  def map_if3
    inject([]){|s,x| (v = yield(x)) ? s << v : s }
  end

  def map_if4
    map{|x| (r = yield(x)) ? r : nil}.compact
  end
end

require 'benchmark'

TIMES = 100
Benchmark.bmbm(10) do |bm|
  arr = (1..10000).to_a
  bm.report("compact!"){ TIMES.times{ arr.map_if{true} } }
  bm.report("select"){ TIMES.times{ arr.map_if2{ true} } }
  bm.report("inject"){ TIMES.times{ arr.map_if3{ true} } }
  bm.report("? : test"){ TIMES.times{ arr.map_if4{ true} } }

  # the block wouldn't let us measure the actual performance:
  #bm.report("compact!"){ TIMES.times{ arr.map_if{|x| x % 7 == 0 and x} } }
  #bm.report("select"){ TIMES.times{ arr.map_if2{|x| x % 7 == 0 and x} } }
  #bm.report("inject"){ TIMES.times{ arr.map_if3{|x| x % 7 == 0 and x} } }
  #bm.report("? : test"){ TIMES.times{ arr.map_if4{|x| x % 7 == 0 and x} } }
end

# >> Rehearsal ---------------------------------------------
# >> compact!    0.380000   0.000000   0.380000 (  0.431046)
# >> select      0.650000   0.000000   0.650000 (  0.725197)
# >> inject      4.300000   0.020000   4.320000 (  4.647794)
# >> ? : test    2.400000   0.010000   2.410000 (  2.551757)
# >> ------------------------------------ total: 7.760000sec
# >> 
# >>                 user     system      total        real
# >> compact!    0.390000   0.000000   0.390000 (  0.408975)
# >> select      0.640000   0.010000   0.650000 (  0.747967)
# >> inject      3.880000   0.010000   3.890000 (  4.211830)
# >> ? : test    2.070000   0.000000   2.070000 (  2.199076)


-- 
Mauricio Fernandez  -   http://eigenclass.org   -  singular Ruby