Feature #7444



Added by marcandre (Marc-Andre Lafortune) almost 9 years ago. Updated almost 4 years ago.

Target version:


I'd like to propose Array#product_set to return the product set of arrays (aka cartesian product)

deck = [1..13, %i(spades hearts diamond clubs)].product_set
    # => <#Enumerator ...>
deck.first(2) # => [[1, :spades], [2, :spades]]

product_set would return an enumerator if no block is given. It should raise an error if an element of the array is not an Enumerable, like Array#transpose or #zip do.

Although Array.product would be acceptable too, I feel that an instance method of array is best in the case, in the same way that transpose is an instance method and not a class method.

The name "product_set" is a correct mathematical term. Although the synonym "cartesian_product" would also be acceptable, I propose "product_set" because it is shorter and cute too. I feel it is even clearer than product; the first time I head of product I was convinced that [2,3,7].product # => 42.

Addressing objections raised in #6499:

1) This is not for the sake of symmetry, but because often we have an array of the arrays we want a product of.

It is cumbersome to write arrays.first.product(*arrays[1..-1]) or similar and it hides what is going on.

Writing arrays.product_set is much nicer.

2) The goal is not mainly to get a lazy version, but more to make the API better. The fact that it returns an Enumerator if no block is given is just a bonus :-)

3) [].product_set.to_a # => [[]]

This can be seen from a cardinality argument, or for example because array.repeated_permutation(n) ==, array).product_set.to_a and array.repeated_permutation(0) == [[]].

Related issues

Has duplicate Ruby master - Feature #8970: and Array.productOpenActions

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