From: Robert Dober <robert.dober@...>
Date: 2006-09-14T02:11:52+09:00
Subject: Re: How to collect over two arrays then look back to one?

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On 9/13/06, Peter Booth <pbooth@marketaxess.com> 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?
>
> Peter


Hmm this seems interestingly complicated - or I am just dully stupid;)

Well here is what I came up with:

a =3D [*1..20]
b =3D  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


--=20
Deux choses sont infinies : l'univers et la b=EAtise humaine ; en ce qui
concerne l'univers, je n'en ai pas acquis la certitude absolue.

- Albert Einstein

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