From: Joel VanderWerf <vjoel@...>
Date: 2006-05-28T04:17:27+09:00
Subject: Re: Basic ruby looping question

Marston A. wrote:
> I'm trying to do something regarding looping through an array, but only 
> with elements I need. I'm not sure exactly how to go about it to make it 
> the most efficient.  Is there basically a way to do something like:
> 
> for row in @large_row_array WHERE row.element == "something"
>  ... some some stuff
> end
> 
> What would be the best way to accomplish the above?  Would I use perhaps 
> some nested loops?  Thanks for any help, I'm just getting started with 
> Ruby along with more indepth programming in general and I'm loving it.

I'm not sure if it's always more efficient, but you can use grep in some
cases:

irb(main):003:0> ENV.keys.grep(/ruby/i) {|key| p key}
"RUBYOPT"
"RUBYLIB"

This is more or less the same as

irb(main):004:0> ENV.keys.each {|key| if /ruby/i === key then p key end}
"RUBYOPT"
"RUBYLIB"

(The return values are quite different though.)

So as long as you are working with tests that can be expressed using
#===, you can use #grep. It's compact, and at least sometimes more
efficient.

require 'benchmark'

a = (0..1_000_000).map {|i| i.to_s}

def foo s
end

Benchmark.bmbm do |b|
  b.report("grep") do
    a.grep(/^10*$/) {|s| foo s}
  end

  b.report("each-if") do
    a.each {|s| if /^10*$/ === s then foo s end }
  end

  b.report("select-each") do
    a.select {|s| /^10*$/ === s}.each {|s| foo s}
  end
end

__END__

Rehearsal -----------------------------------------------
grep          0.490000   0.000000   0.490000 (  0.492667)
each-if       0.640000   0.000000   0.640000 (  0.638102)
select-each   0.760000   0.000000   0.760000 (  0.758676)
-------------------------------------- total: 1.890000sec

                  user     system      total        real
grep          0.490000   0.000000   0.490000 (  0.487539)
each-if       0.640000   0.000000   0.640000 (  0.639301)
select-each   0.620000   0.010000   0.630000 (  0.621152)

-- 
      vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407