irb(main):014:0> books.values.each { |rate| ratings[rate] += 1 }
NoMethodError: undefined method `+' for nil:NilClass
        from (irb):14:in `block in irb_binding'
        from (irb):14:in `each'
        from (irb):14
        from /usr/bin/irb:12:in `<main>'

irb(main):015:0> books.values.each { |rate| ratings[rate] =+ 1 }
=> [:super, :super, :nul, :archi_nul]

irb(main):016:0> ratings
=> {:super=>1, :nul=>1, :archi_nul=>1}

irb(main):017:0> books.values.each { |rate| ratings[rate]+= 1 }
=> [:super, :super, :nul, :archi_nul]

irb(main):018:0> ratings
=> {:super=>3, :nul=>2, :archi_nul=>2}

irb(main):019:0> ratings = {}
=> {}

irb(main):031:0> books.values.each { |rate| ratings[rate]+= 1 }
NoMethodError: undefined method `+' for nil:NilClass
        from (irb):31:in `block in irb_binding'
        from (irb):31:in `each'
        from (irb):31
        from /usr/bin/irb:12:in `<main>'