Given this array of arrays, how do I count the number of arrays that include a duplicate element within itself?

How about this?

arr = [[["Sports", "Soccer"], 2], [["eSports"], 1], [["Sports", "Soccer"], 3]] count = {} arr.each do |el|   el[0].each do |category|     count[category] ||= []     count[category] << el[1]   end end  count.each do |category, ids|   puts "#{category} occurs #{ids.count} times, with Program Ids: #{ids.join(' & ')}" end 

output

Sports occurs 2 times, with Program Ids: 2 & 3 Soccer occurs 2 times, with Program Ids: 2 & 3 eSports occurs 1 times, with Program Ids: 1 

grouped = array.group_by { |s, id| s }.transform_values { |v| v.map(&:last) }  => { ["Sports", "Soccer"] => [2, 3],       ["eSports"]          => [1] } 

To use it:

grouped.each { |k,v|    puts "[#{k.join(', ')}] occurs #{v.length} time(s) with ID: #{v.join(' and ')}"  }  [Sports, Soccer] occurs 2 time(s) with ID: 2 and 3 [eSports] occurs 1 time(s) with ID: 1