Build the cartesian product of multiple arrays in Ruby.

multi_cartesian_product.rb

# Given an array of arrays s = [ [1,2,3], [4,5,6], ... ]
# compute the Cartesian product among the elements of s.

require 'pp'

s = [[1, 2], [3, 4, 5], [6, 7, 8, 9]]
pp s[1..-1].inject(s[0]){ |m,v| m = m.product(v).map(&:flatten) }
[[1, 3, 6],
 [1, 3, 7],
 [1, 3, 8],
 [1, 3, 9],
 [1, 4, 6],
 [1, 4, 7],
 [1, 4, 8],
 [1, 4, 9],
 [1, 5, 6],
 [1, 5, 7],
 [1, 5, 8],
 [1, 5, 9],
 [2, 3, 6],
 [2, 3, 7],
 [2, 3, 8],
 [2, 3, 9],
 [2, 4, 6],
 [2, 4, 7],
 [2, 4, 8],
 [2, 4, 9],
 [2, 5, 6],
 [2, 5, 7],
 [2, 5, 8],
 [2, 5, 9]]

# Or, much simpler (RTFM):

s[0].product(*s[1..-1])

A minor suggestion: In the long form, I think the assignment (m =) is unnecessary in an inject?