# Array1 + array2 newb question **URL:** \ **Category:** ruby-talk\ **Created:** [13 October 2005 08:51 UTC](https://rubytalk.org/t/array1-array2-newb-question/21531 "2005-10-13T08:51:19Z")\ **Posts on this page:** 5\ **Page:** 1

**Author:** ![Kroeger\_Simon\_ext](https://avatars.discourse-cdn.com/v4/letter/k/5daacb/32.png) [@Kroeger\_Simon\_ext](https://rubytalk.org/u/Kroeger_Simon_ext)\ **Post date:** [13 October 2005 08:51 UTC](https://rubytalk.org/t/array1-array2-newb-question/21531/1 "2005-10-13T08:51:19Z")

> From: Robert Klemme [mailto:bob.news@gmx.net] > Since there hasn't been an #inject solution so far... 🙂 hmm, is the gateway broken again? a1.inject(){|s, x| s + a2.map {|y| [x, y]}} (posted 1 hour ago) cheers Simon ---

**Author:** ![Robert](https://avatars.discourse-cdn.com/v4/letter/r/e95f7d/32.png) [@Robert](https://rubytalk.org/u/Robert)\ **Post date:** [13 October 2005 09:46 UTC](https://rubytalk.org/t/array1-array2-newb-question/21531/2 "2005-10-13T09:46:54Z")

Kroeger, Simon (ext) wrote: > > From: Robert Klemme [mailto:bob.news@gmx.net] > > Since there hasn't been an #inject solution so far... 🙂 > > hmm, is the gateway broken again? > > a1.inject(){|s, x| s + a2.map {|y| [x, y]}} > > (posted 1 hour ago) No, it's here. Can't tell though whether I just overlooked it or whether the posting arrived here after I wrote mine. And note that your version has the disadvantage that you copy s all the time. Using concat is more efficient. Even if you use that there's an intermediate array per x value that is dropped again. That's why my solution just appends two element arrays to the collecting array. 🙂 Kind regards robert ---

**Author:** ![daz](https://avatars.discourse-cdn.com/v4/letter/d/87869e/32.png) [@daz](https://rubytalk.org/u/daz)\ **Post date:** [13 October 2005 12:56 UTC](https://rubytalk.org/t/array1-array2-newb-question/21531/3 "2005-10-13T12:56:56Z")

Robert Klemme wrote: > Kroeger, Simon (ext) wrote: > \>\> From: Robert Klemme [mailto:bob.news@gmx.net] > \>\> Since there hasn't been an #inject solution so far... 🙂 > \> > \> hmm, is the gateway broken again? > \> > \> a1.inject(){|s, x| s + a2.map {|y| [x, y]}} > \> > \> (posted 1 hour ago) > > No, it's here. Can't tell though whether I just overlooked it > or whether the posting arrived here after I wrote mine. > > And note that your version has the disadvantage [...] A more significant disadvantage is that you both posted solutions with equivalent results but for a slightly different problem than the one posed by the OP 😉 > [...] that you copy s all the time. As did my solution. > Using concat is more efficient. That's usually safe to say ... but check the b/marks. > That's why my solution just appends two element arrays to > the collecting array. 🙂 .... And ends up 3rd out of 3 in the b/mk race 😃 > Kind regards > > robert After making the necessary corrections: > **···** > > #-------------------- > require 'benchmark' > include Benchmark > > a1 = [1,2,3] > a2 = %w{a b c} > > BMN = 50000 # \<------ ?whatever? > bm(7) do |bx| > bx.report('d-') { BMN.times { dz = ; a2.each{|s| dz += a1.map {|y| [y, s]}} }} > bx.report('s-') { BMN.times { sk = a2.inject(){|s,x| s + a1.map {|y| [y, x]}} }} > bx.report('r-') { BMN.times { rk = a2.inject(){|r,x| a1.each {|y| r \<\< [y,x]}; r} }} > GC.start ; GC.disable > bx.report('dz') { BMN.times { dz = ; a2.each{|s| dz += a1.map {|y| [y, s]}} }} > GC.start ; GC.disable > bx.report('sk') { BMN.times { sk = a2.inject(){|s,x| s + a1.map {|y| [y, x]}} }} > GC.start ; GC.disable > bx.report('rk') { BMN.times { rk = a2.inject(){|r,x| a1.each {|y| r \<\< [y,x]}; r} }} > end > > =begin > user system total real > d- 5.857000 0.000000 5.857000 ( 5.880000) > s- 7.145000 0.000000 7.145000 ( 7.140000) > r- 8.040000 0.000000 8.040000 ( 8.070000) > dz 5.415000 0.000000 5.415000 ( 5.440000) > sk 6.428000 0.000000 6.428000 ( 6.420000) > rk 7.353000 0.000000 7.353000 ( 7.360000) > =end > #-------------------- > > IMHO - #inject is very useful but I don't see the advantage > of using it to hide an initial x = and > subjecting it to being flung round in circles. > > As you can see, it doesn't gain anything, here. > > (Not a squabble 🙂 > > daz ---

**Author:** ![Robert](https://avatars.discourse-cdn.com/v4/letter/r/e95f7d/32.png) [@Robert](https://rubytalk.org/u/Robert)\ **Post date:** [13 October 2005 15:21 UTC](https://rubytalk.org/t/array1-array2-newb-question/21531/4 "2005-10-13T15:21:54Z")

daz wrote: > Robert Klemme wrote: > A more significant disadvantage is that you both posted solutions > with equivalent results but for a slightly different problem than > the one posed by the OP 😉 Right. Shouldn't make a difference performance wise though. > > That's why my solution just appends two element arrays to > > the collecting array. 🙂 > > ... And ends up 3rd out of 3 in the b/mk race 😃 It depends... (see below) > IMHO - #inject is very useful but I don't see the advantage > of using it to hide an initial x = and > subjecting it to being flung round in circles. > > As you can see, it doesn't gain anything, here. > > (Not a squabble 🙂 Your benchmark version crashed on my system because it used too much mem. IMHO GC times should be \*included\* because in a real world situation the time overhead incurred by massive object allocations and deallocations does in fact influence performance. However, with a modified script I get 17:05:03 [source]: ruby /c/temp/ruby/mappings.rb Rehearsal ------------------------------------------ dz 5.719000 0.000000 5.719000 ( 5.733000) dzco 4.984000 0.000000 4.984000 ( 4.988000) sk 6.891000 0.000000 6.891000 ( 6.959000) co 6.187000 0.000000 6.187000 ( 6.193000) rk 5.625000 0.000000 5.625000 ( 5.674000) rk2 8.938000 0.000000 8.938000 ( 9.017000) rk3 9.000000 0.000000 9.000000 ( 9.074000) -------------------------------- total: 47.344000sec user system total real dz 5.719000 0.000000 5.719000 ( 5.762000) dzco 4.921000 0.000000 4.921000 ( 4.997000) sk 6.875000 0.000000 6.875000 ( 6.918000) co 6.234000 0.000000 6.234000 ( 6.270000) rk 5.641000 0.000000 5.641000 ( 5.669000) rk2 9.078000 0.000000 9.078000 ( 9.137000) rk3 9.078000 0.000000 9.078000 ( 9.118000) Interestingly the double inject performs so bad. However the one inject version is approx as fast as "dz". But the clear winner is \*surprise surprise\* dz = ; a2.each{|s| dz.concat a1.map {|y| [y, s]}} Would be interesting to see how timings differ for larger arrays... 17:15:43 [source]: ruby /c/temp/ruby/mappings-2.rb 96 Rehearsal ------------------------------------------ dz 3.375000 0.015000 3.390000 ( 3.405000) dzco 3.485000 0.000000 3.485000 ( 3.532000) sk 3.453000 0.000000 3.453000 ( 3.478000) co 3.532000 0.000000 3.532000 ( 3.581000) rk 3.984000 0.000000 3.984000 ( 4.030000) rk2 6.594000 0.000000 6.594000 ( 6.619000) rk3 6.640000 0.000000 6.640000 ( 6.663000) -------------------------------- total: 31.078000sec user system total real dz 3.344000 0.016000 3.360000 ( 3.370000) dzco 3.157000 0.000000 3.157000 ( 3.192000) sk 3.422000 0.000000 3.422000 ( 3.429000) co 3.203000 0.000000 3.203000 ( 3.240000) rk 3.266000 0.000000 3.266000 ( 3.256000) rk2 6.562000 0.000000 6.562000 ( 6.576000) rk3 7.031000 0.000000 7.031000 ( 7.117000) Differences are much larger. Still I have no clue why the double inject version is so slow... Kind regards robert [mappings.rb](https://rubytalk.org/uploads/short-url/b0Az6w00ezmaApJWnxDe6VoHJGv.rb) (768 Bytes) [mappings-2.rb](https://rubytalk.org/uploads/short-url/819q4D3ueXjdY8JUUKQ9mxJfyfn.rb) (818 Bytes) ---

**Author:** ![daz](https://avatars.discourse-cdn.com/v4/letter/d/87869e/32.png) [@daz](https://rubytalk.org/u/daz)\ **Post date:** [13 October 2005 22:31 UTC](https://rubytalk.org/t/array1-array2-newb-question/21531/5 "2005-10-13T22:31:59Z")

Robert Klemme wrote: > daz wrote: > \> Robert Klemme wrote: > > dz = ; a2.each{|s| dz.concat a1.map {|y| [y, s]}} That's enlightening about #concat. When you mentioned it in reply to Simon, I read it as a reference to your use of \<\<. #concat is one of those methods that wasn't in my vocabulary -- I thought it was an alias for + or something ... not so. Most of the time I use \<\< unless it causes unrequired nesting of arrays, then I go for +. What I hoped I was doing with + is what #concat does, i.e. splices to the receiving array without creating a new one. As Douglas Adams almost wrote: "Well, that just about wraps it up for Array#+" 😉 Cheers, daz + ....... Concatenation (produces a third array) concat .. Appends elements (think of as +! bang-method) push .... \<\< \<\< ...... Append object (alias of push)