From: William James <w_a_x_man@...>
Date: 2006-09-14T06:20:41+09:00
Subject: Re: How to collect over two arrays then look back to one?


Peter Booth 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] ))
}