Computing frequency of terms in a multidimensional array in Ruby

Question

I have an array : keys=[["a","a","b"],["a","b","c"]] I need to find the number of times "a","b","c" occurs in each sub-array of 'keys'.

output could be a hash : ["a"=> [2,1],"b"=>[1,1],"c"=>[0,1]]

Answer 1

Perhaps not the fastest, but probably among the shortest:

Hash[
  keys.flatten.uniq.map{|e|
    [e, keys.map{|ar| ar.count(e)}]
  }
]
=> {"a"=>[2, 1], "b"=>[1, 1], "c"=>[0, 1]}

or

keys.flatten.uniq.inject({}){|acc,e|
  acc.merge({e => keys.map{|ar| ar.count(e)}})
}

Here's a shot at 1.8.6 version:

keys.flatten.uniq.inject({}){|acc,e|
  acc[e] = keys.map{|ar|
    ar.select{|c| c==e}.size
  }
  acc
}

But you'd better get that backports gem soon... ;)