Like map, but for two lists instead of one.

map2.rb

#
# Combines 2 vectors into a single vector of size av, 
# by applying a binary operation (defined in block) 
# on each element.
#
# Examples:
#   map2([1,2,3], [4,5,6]) { |a,b| a*b } == [4,10,18]
#   map2([1,2,3], [4,5]  ) { |a,b| a+b } == [5,7,nil]
#   map2([1,2],   [4,5,6]) { |a,b| a+b } == [5,7]
#
def map2(av, bv, &block)
  raise 'Argument is not an Enumerable' \
    if !av.kind_of?(Enumerable) || !bv.kind_of?(Enumerable)
  result = Array.new av.size
  av.size.times do
    i, size = 0, av.size
    while i < size
      result[i] = i < bv.size ? 
        yield(av[i], bv[i]) : nil
      i += 1
    end
  end
  result
end