有点奇怪

Take a look of a point access option of data set. Here is an example.

data x;
input X $1.;
cards;
a
b
c
a
b
a
b
c
a
b
;

data y;
  set x nobs=nobs end=end;
  cnt=_n_+1;
  if cnt<nobs then set x(rename=(x=x2)) point=cnt;
  output;
  if x='b' and x2 ne 'c'  then do;
     x='c';
         output;
  end;
  keep x;
run;
proc print;run;

thanks very much