From: Mike Harris <GENIE@...>
Date: 2006-11-16T04:08:48+09:00
Subject: Re: Design problem with 'inject'

Jacob Fugal wrote:

> On 11/14/06, Gary Boone <dr@garyboone.com> wrote:
>
>>    arr.inject(0) do |sum, i|
>>                    next if (i==3)
>>                    sum += i
>>                  done
>
>
> Others have already posted clarifying the semantics of inject and
> proposing alternate solutions, so I won't bother with that. I just
> wanted to point out another alternate:
>
>  arr.select{ |i| i != 3 }.inject(0){ |sum, i| sum + i }
>
> I like to use block-taking functions on Enumerables like unix pipes
> and let the body of each block do the simplest thing possible.
>
> The only potential drawback to breaking the filter apart like this is
> that the current implementation of ruby turns this into two loops. If
> your array is large, this can be a problem. But for most cases where
> you're not doing number crunching, the inefficiency is negligible.
>
> Just another possibility...
>
> Jacob Fugal
>
>
Agree 100% on the use of select

On performance, it depends on what you're doing.  The more items you 
want to exclude, the better select does.

tm is just a method to time the block

tm("One Loop") { a.inject { |s,i| (false) ? s+i : s } }
tm("Two Loops") { a.select { |x| false }.inject(0) { |s,i| s+i } }
Timing One Loop       : 2 Seconds 188 Milliseconds
Timing Two Loops      : 0 Seconds 327 Milliseconds

tm("One Loop") { a.inject { |s,i| (x%10==0) ? s+i : s } }
tm("Two Loops") { a.select { |x| x%10==0 }.inject(0) { |s,i| s+i } }
Timing One Loop       : 3 Seconds 32 Milliseconds
Timing Two Loops      : 1 Second 234 Milliseconds

tm("One Loop") { a.inject { |s,i| (x%2==0) ? s+i : s } }
tm("Two Loops") { a.select { |x| x%2==0 }.inject(0) { |s,i| s+i } }
Timing One Loop       : 3 Seconds 907 Milliseconds
Timing Two Loops      : 3 Seconds 188 Milliseconds

tm("One Loop") { a.inject { |s,i| (x%10!=0) ? s+i : s } }
tm("Two Loops") { a.select { |x| x%10!=0 }.inject(0) { |s,i| s+i } }
Timing One Loop       : 5 Seconds 48 Milliseconds
Timing Two Loops      : 5 Seconds 235 Milliseconds

tm("One Loop") { a.inject { |s,i| (true) ? s+i : s } }
tm("Two Loops") { a.select { |x| true }.inject(0) { |s,i| s+i } }
Timing One Loop       : 4 Seconds 532 Milliseconds
Timing Two Loops      : 5 Seconds 314 Milliseconds