# How to collect over two arrays then look back to one? **URL:** \ **Category:** Ruby\ **Created:** [September 13, 2006, 6:11pm UTC](https://www.ruby-forum.com/t/how-to-collect-over-two-arrays-then-look-back-to-one/68571 "2006-09-13T18:11:47Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![bodikp](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/e79b87/32.png) [@bodikp](https://www.ruby-forum.com/u/bodikp)\ **Post date:** [September 13, 2006, 6:11pm UTC](https://www.ruby-forum.com/t/how-to-collect-over-two-arrays-then-look-back-to-one/68571/1 "2006-09-13T18:11:47Z")
I am reading Ruby for Rails and Ruby Cookbock and delighted by the Array#find\_all and Enumerable#collect methods. I’m wondering what the idiomatic way to do the following is? Two parallel arrays A, B . For each element n of A for which f(An) is true, is g(B[n-3:n+3]) true? Where B[n-3:n+3] refers to seven elements of B whose position is centered over An’ position. Can the block inside Enumerable#collect know it’s position inside the Enumerable? Can one collect two parallel arrays? ## Peter * * * The information contained in and accompanying this communication is strictly confidential and intended solely for the use of the intended recipient(s). If you have received it by mistake please let us know by reply and then delete it from your system; you should not copy the message or disclose its content to anyone. MarketAxess reserves the right to monitor the content of emails sent to or from its systems. Any comments or statements made are not necessarily those of MarketAxess. For more information, please visit [www.marketaxess.com](http://www.marketaxess.com). MarketAxess Europe Limited is regulated in the UK by the FSA, registered in England no. 4017610, registered office at 71 Fenchurch Street, London, EC3M 4BS. Telephone (020) 7709 3100. MarketAxess Corporation is regulated in the USA by the SEC and the NASD, incorporated in Delaware, executive offices at 140 Broadway, New York, NY 10005. Telephone (1) 212 813 6000. ---
**Author:** ![bodikp](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/e79b87/32.png) [@bodikp](https://www.ruby-forum.com/u/bodikp)\ **Post date:** [September 13, 2006, 7:11pm UTC](https://www.ruby-forum.com/t/how-to-collect-over-two-arrays-then-look-back-to-one/68571/2 "2006-09-13T19:11:56Z")
On 9/13/06, Peter B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Can the block inside Enumerable#collect know it’s position inside the > Enumerable? Can one collect two parallel arrays? > > Peter Hmm this seems interestingly complicated - or I am just dully stupid;) Well here is what I came up with: a = [\*1…20] b = a.map{ |e| “Line #{e}” } def ranges a, b, n, &block ### The following is what you were asking for 😉 ``` b.values_at(*(0..a.length-1).zip(a).map{|e| block.call(e.last) ? ``` e.first: nil}. compact.map{|i| [\*(i-n)…(i+n)]}.flatten.uniq.reject{|i|i\<0}).compact end p ranges(a,b,1) { |e| (e%3).zero? } p ranges(a,b,2) { |e| (e%3).zero? } p ranges(a,b,1) { |e| (e%5).zero? } p ranges(a,b,2) { |e| (e%5).zero? } HTH Robert PS: Golfers cut it down 😉 R – Deux choses sont infinies : l’univers et la bêtise humaine ; en ce qui concerne l’univers, je n’en ai pas acquis la certitude absolue. - Albert Einstein ---
**Author:** ![bodikp](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/e79b87/32.png) [@bodikp](https://www.ruby-forum.com/u/bodikp)\ **Post date:** [September 13, 2006, 7:13pm UTC](https://www.ruby-forum.com/t/how-to-collect-over-two-arrays-then-look-back-to-one/68571/3 "2006-09-13T19:13:56Z")
I think… a.values\_at(\*(0…a.size).to\_a.select {|i| f(a[i]) && g([i-3,0].max…[i+3,a.size-1].min)}) Ruby seems a little verbose on this one. -j ---
**Author:** ![bodikp](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/e79b87/32.png) [@bodikp](https://www.ruby-forum.com/u/bodikp)\ **Post date:** [September 13, 2006, 10:16pm UTC](https://www.ruby-forum.com/t/how-to-collect-over-two-arrays-then-look-back-to-one/68571/4 "2006-09-13T22:16:25Z")
On 9/13/06, Jason N. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > I think… > > a.values\_at(\*(0…a.size).to\_a.select {|i| f(a[i]) && g([i-3,0].max…[i+3, a.size-1].min)}) I am afraid this does not what you want, select returns i’s not the value of the block you had some great ideas I have stolen below though 🙂 I’ll try to fix your interpretation, ah I see you think he just wants the indicies, so your solution is (almost) great, you can use [\*0…a.size] but you might have a problem with f(nil) so use [_0…a.size-1] OP also stated he wanted g(b[something]) not g(something) it is a riddle an alternative might be b.values\_at([0…a.size-1].select{|i|f a[i]}.map{|i| g(b.values\_at([_(i-n)…(i+n)].reject etc. etc. Ruby seems a little verbose on this one. > -j I still think OP thaught g to be something abstract so I would write def ranges a, b, n, &block ### The following is what you were asking for 😉 ``` b.values_at( *(0..a.length-1).select{|i| block.call(a[i]) }.map{|i| ``` [\*(i-n)…(i+n)] }. flatten.uniq.reject{|i|i\<0}).compact end thanks to your ideas letting me see my stupid mistakes… Robert Peter B. wrote: > > Can the block inside Enumerable#collect know it’s position inside the > > recipient(s). > > Europe – Deux choses sont infinies : l’univers et la bêtise humaine ; en ce qui concerne l’univers, je n’en ai pas acquis la certitude absolue. - Albert Einstein ---
**Author:** ![bodikp](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/e79b87/32.png) [@bodikp](https://www.ruby-forum.com/u/bodikp)\ **Post date:** [September 13, 2006, 11:20pm UTC](https://www.ruby-forum.com/t/how-to-collect-over-two-arrays-then-look-back-to-one/68571/5 "2006-09-13T23:20:33Z")
Peter B. wrote: > I am reading Ruby for Rails and Ruby Cookbock and delighted by the > Array#find\_all and Enumerable#collect methods. I’m wondering what the > idiomatic way to do the following is? > > Two parallel arrays A, B . For each element n of A for which f(An) is true, > is g(B[n-3:n+3]) true? Where B[n-3:n+3] refers to seven elements of B whose > position is centered over An’ position. > > Can the block inside Enumerable#collect know it’s position inside the > Enumerable? Can one collect two parallel arrays? a = Array.new(12) {|i| i_i} b = Array.new(12) {|i| i_3} def f(n) 1 == n % 2 end def g(a) return false if a.size != 7 0 == a.inject{|p,n| p\*n}/a.inject{|s,n| s+n} % 4 end result = [] a.each\_with\_index{ |e,i| result \<\< (f(e) && g( b[i-3 … i+3] )) } ---
**Author:** ![bodikp](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/e79b87/32.png) [@bodikp](https://www.ruby-forum.com/u/bodikp)\ **Post date:** [September 13, 2006, 11:22pm UTC](https://www.ruby-forum.com/t/how-to-collect-over-two-arrays-then-look-back-to-one/68571/6 "2006-09-13T23:22:33Z")
Peter B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes: > I am reading Ruby for Rails and Ruby Cookbock and delighted by the > Array#find\_all and Enumerable#collect methods. I’m wondering what the > idiomatic way to do the following is? Kind of. But first I’m going to re-arrange your email to answer more didactically. 🙂 > Can the block inside Enumerable#collect know it’s position inside the > Enumerable? Enumerable::Enumerator – require ‘enumerator’ – can help you out with this. It lets you create an Enumerable object which can call any ‘each’-like method of any other object. You can in turn call the ‘collect’ method of the Enumerator object. So: ``` array.to_enum(:each_with_index).collect { |value, index| ... } ``` (Using :each\_with\_index is a common enough case that Enumerator also adds #enum\_with\_index to Enumerable.) > Can one collect two parallel arrays? In several ways… The most basic is the Enumerable#zip method: ``` (0..3).zip(('a'..'d')) # => [[0, "a"], [1, "b"], [2, "c"], [3, "d"]] ``` It returns a new array mapping each element of the receiving Enumerable to the corresponding elements of the Enumerables passed as arguments. > Two parallel arrays A, B . For each element n of A for which f(An) is true, > is g(B[n-3:n+3]) true? Where B[n-3:n+3] refers to seven elements of B whose > position is centered over An’ position. Enumerable#inject is the most idiomatic way to perform most Enumerable iteration not directly supported by other methods. If I understand you properly, in this case you probably want something like: ``` a.enum_with_index.inject([]) do |result, (v, n)| result << (f(v) ? g(b[n-3..n+3]) : false) end ``` Hope this helps! -Marshall ---
**Author:** ![bodikp](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/e79b87/32.png) [@bodikp](https://www.ruby-forum.com/u/bodikp)\ **Post date:** [September 15, 2006, 11:15am UTC](https://www.ruby-forum.com/t/how-to-collect-over-two-arrays-then-look-back-to-one/68571/7 "2006-09-15T11:15:31Z")
removed\_email\_address@domain.invalid writes: > To answer the first question: no. You are free to join Citizens for > map\_with\_index, of which I am the founder and president 🙂 What about Citizens Who Are Happy With enum\_with\_index.map ? I think they have a thing-or-two to say on this issue… -Marshall ---
**Author:** ![bodikp](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/e79b87/32.png) [@bodikp](https://www.ruby-forum.com/u/bodikp)\ **Post date:** [September 15, 2006, 10:45am UTC](https://www.ruby-forum.com/t/how-to-collect-over-two-arrays-then-look-back-to-one/68571/8 "2006-09-15T10:45:30Z")
Hi – On Thu, 14 Sep 2006, Peter B. wrote: > I am reading Ruby for Rails and Ruby Cookbock and delighted by the > Array#find\_all and Enumerable#collect methods. I’m wondering what the > idiomatic way to do the following is? > > Two parallel arrays A, B . For each element n of A for which f(An) is true, > is g(B[n-3:n+3]) true? Where B[n-3:n+3] refers to seven elements of B whose > position is centered over An’ position. > > Can the block inside Enumerable#collect know it’s position inside the > Enumerable? Can one collect two parallel arrays? To answer the first question: no. You are free to join Citizens for map\_with\_index, of which I am the founder and president 🙂 David ---
**Author:** ![bodikp](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/e79b87/32.png) [@bodikp](https://www.ruby-forum.com/u/bodikp)\ **Post date:** [September 15, 2006, 3:16pm UTC](https://www.ruby-forum.com/t/how-to-collect-over-two-arrays-then-look-back-to-one/68571/9 "2006-09-15T15:16:17Z")
On Fri, Sep 15, 2006 at 06:15:59PM +0900, Marshall T. Vandegrift wrote: > removed\_email\_address@domain.invalid writes: > > > To answer the first question: no. You are free to join Citizens for > > map\_with\_index, of which I am the founder and president 🙂 > > What about Citizens Who Are Happy With enum\_with\_index.map ? I think they > have a thing-or-two to say on this issue… > > -Marshall Here, here! Down with the giant collection of methods! Up with higher order methods!