X

                 Y

                 1

                 2

                 1

                 5

                 1

                 4

                 2

                 3

                 2

                 2

第一步，sort x y              //在X组内把Y按照大小排列

                 X

                 Y

                 1

                 2

                 1

                 4

                 1

                 5

                 2

                 2

                 2

                 3

第二步，gen n=_n          //生成一个新变量n=1,2,3,4,5

                gen N=_N        //生成一个新变量N=5,5,5,5,5

                list

                X

                Y

                n

                N

                 1

                 2

            1

               5

                 1

                 4

            2

               5

                 1

                 5

            3

               5

                 2

                 2

            4

               5

                 2

                 3

            5

               5

第三步，by x, sort: gen n1=_n           //n1按照x进行分类操作

                by x, sort: gen N1=_N          

                list

          X

          Y

          n

          N

         n1

        N1

                 1

                 2

            1

          5

          1

        3

                 1

                 4

            2

          5

          2

        3

                 1

                 5

            3

          5

          3

        3

                 2

                 2

            4

          5

          1

        2

                 2

                 3

            5

          5

          2

        2