Finding the product of a variable number of Ruby arrays

Question

I'm looking to find all combinations of single items from a variable number of arrays. How do I do this in Ruby?

Given two arrays, I can use Array.product like this:

groups = []
groups[0] = ["hello", "goodbye"]
groups[1] = ["world", "everyone"]

combinations = groups[0].product(groups[1])

puts combinations.inspect 
# [["hello", "world"], ["hello", "everyone"], ["goodbye", "world"], ["goodbye", "everyone"]]

How could this code work when groups contains a variable number of arrays?

Jörg W Mittag · Accepted Answer · 2010-08-05 23:39:26Z

14

groups = [
  %w[hello goodbye],
  %w[world everyone],
  %w[here there]
]

combinations = groups.first.product(*groups.drop(1))

p combinations
# [
#   ["hello", "world", "here"],
#   ["hello", "world", "there"],
#   ["hello", "everyone", "here"],
#   ["hello", "everyone", "there"],
#   ["goodbye", "world", "here"],
#   ["goodbye", "world", "there"],
#   ["goodbye", "everyone", "here"],
#   ["goodbye", "everyone", "there"]
# ]

edited Aug 5, 2010 at 23:39

answered Aug 5, 2010 at 23:34

Jörg W Mittag

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5 Comments

Ollie Glass Over a year ago

Wow, thank you. Could you, or someone, possibly explain how this works?

jtbandes Over a year ago

An explanation of what this actually does would be helpful too, and probably lead to better design of the OP's code...

jtbandes Over a year ago

@Ollie: Array#product can take multiple arrays as arguments, so this is just basically doing groups[0].product(groups[1],groups[2],...)

Chuck Over a year ago

How it works: product takes however many arrays you apply to it and gives the Cartesian product of the receiver and all the arguments. The splat operator "expands" an array into a list of arguments, so we take all the arrays in groups except the first and pass them as arguments to product.

lightswitch05 Over a year ago

One note, if your groups array is truly variable, you have to take into account when it is empty and when it only has 1 array, otherwise you might get undefined method 'product' for nil:NilClass

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