如果 sas 数据集中包含 3 个观察值，如何交换第一个和最后一个观察值

Question

我有一个包含 3 个观察值的数据集，1 2 3 4 5 6 7 8 9，现在我必须交换 1 2 3 和 7 8 9。如何在基础 sas 中做到这一点？

Answer 1

If you just want to sort your dataset by a variable in descending order, use proc sort:

data example;
  input number;
  datalines;
123
456
789
;
run;

proc sort data = example;
    by descending number;
run;

If you want to re-order a dataset in a more complex way, create a new variable containing the position that you want each row to be in, and then sort it by that variable.

Answer 2

If you want to swap the contents of the first and last observations while leaving the rest of the dataset in place, you could do something like this.

data class;
  set sashelp.class;
run;

data firstobs;
  i = 1;
  set sashelp.class(obs = 1);
run;


data lastobs; 
  i = nobs;
  set sashelp.class nobs = nobs point = nobs;
  output;
  stop;
run;

data transaction;
  set lastobs firstobs;
  /*Swap the values of i for first and last obs*/
  retain _i;
  if _n_ = 1 then do;
    _i = i;
    i = 1;
  end;
  if _n_ = 2 then i = _i;
  drop _i;
run;

data class;
  set transaction(keep = i);
  modify class point = i;
  set transaction;
run;

This modifies just the first and last observations, which should be quite a bit faster than sorting or replacing a large dataset. You can do a similar thing with the update statement, but that only works if your dataset is already sorted / indexed by a unique key.

Answer 3

By Sandeep Sharma:sandeep.sharmas091@gmail.com

  data testy;
     input a; 
     datalines;
1
2
3
4
5
6
7
8
9
;
run;

data ghj;
      drop y;
         do i=nobs-2 to nobs;
           set testy point=i nobs=nobs;
             output;
               end;

        do n=4 to nobs-3; 
          set testy point=n;
            output;
              end;

           do y=1 to 3;
             set testy;
               output;
                 end;
stop;
run;