Question

I have a hash like this:

h={
  :p1 => [:c1,:c2],
  :p2 => [:c3,:c4],
  :p3 => [:c1,:c5],
  :p4 => [:c3,:c2],
  :p5 => [:c6,:c7]
} 

I’d like to find a nice Ruby function that returns an array of arrays like this:

res=[[:p1,:p3],[:p2,:p4]]

Each sub-array is made of hash keys whose first values associated are equal: :p1 and :p3 have :c1 as a starting value, and :p2 and :p4 have :c3.

Answer 1

How’s this?

h.keys.group_by {|k| h[k][0]}
# => {:c1=>[:p1, :p3], :c3=>[:p2, :p4], :c6=>[:p5]}