From: "Marshall T. Vandegrift" <llasram@...>
Date: 2006-09-14T06:22:52+09:00
Subject: Re: How to collect over two arrays then look back to one?

Peter Booth <pbooth@marketaxess.com> 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