From: Kaspar Schiess <eule@...>
Date: 2004-04-22T20:28:43+09:00
Subject: Re: efficient collect+compact ?

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Martin Pirker wrote:

| I read Array and Enumerable docs, have I missed something[1] or does
| a straight "collect everything but nil" function not exist?

The function select collects everything but false elements, the function
reject does the opposite. I am surprised that this did not come up as
the first answer:

(insert this into code posted earlier on thread)
b.report 'select+collect' do
~  ary= Z.select{ |x| x%2 != 0 }.collect{|x| x+10}
~  p ary[0]
~ end
~ b.report 'reject+collect' do
~  ary= Z.reject{ |x| x%2 == 0 }.collect{|x| x+10}
~  p ary[0]
~ end

Performance comparison yields:
~      user     system      total        real
map!+compact!
~  1.202000   0.000000   1.202000 (  1.422000)
map+compact
~  0.651000   0.010000   0.661000 (  0.711000)
inject
~  5.378000   0.000000   5.378000 (  5.608000)
external
~  0.781000   0.000000   0.781000 (  0.781000)
external for
~  0.741000   0.000000   0.741000 (  0.741000)
select+collect
~  0.871000   0.000000   0.871000 (  0.872000)
reject+collect
~  0.831000   0.000000   0.831000 (  0.831000)

for a million elements. Ranking as follows (from this one test):
map+compact
external for
external
reject+collect
select+collect
map!+compact!
inject

I take the following lessons home:
- - map/collect is fast, since it preallocates arrays.
- - external for is fast, no allocating of blocks
- - inject is evil because of its implementation

ruby 1.8.1 (2004-01-27) [i386-mswin32] btw.

Anyone more explanations ?

kaspar - code philosopher

- -- stolen off the net --
A rock pile ceases to be a rock pile the moment a single man
contemplates it, bearing within him the image of a cathedral.
~                -- Antoine de Saint-Exupery
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