Question

I need this Hash

{"peter" => ["apple", "orange", "mango"], "sandra" => ["flowers", "bike"]}

convert to this Array:

[["peter", "apple"], ["peter", "orange"], ["peter", "mango"], ["sandra", "flowers"], ["sandra", "bike"]]

Now I have got this solution

my_hash.inject([]){|ar, (k,v)| ar << v.map{|c| [k,c]}}.flatten(1)

But I believe here is more elegant solution with those zip or transpose magick 🙂

Answer 1

You are right to be suspicious about Enumerable#inject solutions. In Ruby, inject/reduce is somewhat abused, we must be careful and choose the right abstraction (map, select, zip, flatten…) if they fit the problem at hand. In this case:

h = {"peter" => ["apple", "orange", "mango"], "sandra" => ["flowers", "bike"]}
h.map { |k, vs| vs.map { |v| [k, v] } }.flatten(1)
#=> [["peter", "apple"], ["peter", "orange"], ["peter", "mango"], ["sandra", "flowers"], ["sandra", "bike"]]

But if you want to use Enumerable#zip don’t let anyone stop you 😉

h.map { |k, vs| [k].cycle(vs.size).zip(vs) }.flatten(1)

And as @steenslag says, also:

h.map { |k, vs| [k].product(vs) }.flatten(1)

So at the end we can write:

h.flat_map { |k, vs| [k].product(vs) }