words = ["foo", "var", "spam", "egg", "foo", "foo", "egg"]
    frequency = {}
    words.each{|word| frequency[word] += 1}

puts(frequency)   #{"foo" => 3, "var" => 1, "spam" => 1, "egg" => 2}

prueba.rb:3:in `block in <main>': undefined method `+' for nil:NilClass (NoMethodError)