From: zoranlazarevic@... (Zoran Lazarevic)
Date: 2003-10-07T08:23:13+09:00
Subject: map/collect iterating over multiple arrays/arguments

Can I iterate over multiple arrays/collections?

It is very useful (and used in Lisp a lot) to map several lists onto
one, like map/collect but iterating over multiple collections at the
same time. For example:

    foods   = ['banana', 'grass', 'peanuts']
    animals = ['monkey', 'gnu',   'elephant']
    
    [animals,foods].multi_each{|x,y| puts "#{x} eats #{y}" }

money eats banana
gnu eats grass
elephant eats peanuts

    [animals,foods].multi_map{|x,y| "#{x} eats #{y}" }

=> ["money eats banana", "gnu eats grass", "elephant eats peanuts"]


This is relatively simple to implement using current Ruby features,
but at the cost of either zipping all input collections in memory, or
requiring that array implements integer indexing:


foods.zip(animals).each{|x,y| puts "#{x} eats #{y}" }

or

module Enumerable
    def multi_map
        result = []
        self[0].each_index{|i|
            args = self.collect{|x| x[i]}
            result << yield(*args)
        }
        result
    end
end

I assume this can also be done by using continuation, but is there a
simpler way. Are there Iterators for enumerables, like in Java?

--Laza

Zoran Lazarevic
212.569.4011
http://www.cs.columbia.edu/~laza