/*<pre><b>
/ Program   : lookahead.sas
/ Version   : 1.0
/ Author    : Roland Rashleigh-Berry
/ Date      : 13-Feb-2007
/ Purpose   : To do the opposite of lag and allow the user to look ahead at the
/             variables in the following observations in the same by group.
/ SubMacros : %words
/ Notes     : The input dataset must be sorted into the correct order. The
/             "bygroup" parameter will be set to the grouping variables which is
/             not the same as the list of sort variables since the records will
/             be sorted WITHIN the group as well. Suppose your sort variables 
/             were "subject, date, time" then your "bygroup" variable might just
/             be "subject". The look-ahead is only done within the "bygroup" set.
/             
/             The lookahead variables created will have the same name as the 
/             original variables except with a numeric suffix on the end. So for
/             the observation following it will have the suffix "1" and for the
/             second observation it will have the suffix "2" etc.
/ Usage     : 
/ 
/===============================================================================
/ PARAMETERS:
/-------name------- -------------------------description------------------------
/ dsin=             Input dataset
/ dsout=            Output dataset
/ bygroup=          Grouping variables(s)
/ vars=             Variables you want to look ahead for
/ lookahead=1       Number of observations to look ahead
/===============================================================================
/ AMENDMENT HISTORY:
/ init --date-- mod-id ----------------------description------------------------
/ rrb  13Feb07         "macro called" message added
/===============================================================================
/ This is public domain software. No guarantee as to suitability or accuracy is
/ given or implied. User uses this code entirely at their own risk.
/==============================================================================*/

%put MACRO CALLED: lookahead v1.0;

%macro lookahead(dsin=,dsout=,bygroup=,vars=,lookahead=1);

%local error i;
%let error=0;


            /*--------------------------------------*
               Check we have enough parameters set
             *--------------------------------------*/

%if not %length(&dsin) %then %do;
  %let error=1;
  %put ERROR: (lookahed) Nothing specified to dsin= parameter;
%end;

%if not %length(&bygroup) %then %do;
  %let error=1;
  %put ERROR: (lookahed) Nothing specified to bygroup= parameter;
%end;

%if not %length(&vars) %then %do;
  %let error=1;
  %put ERROR: (lookahed) Nothing specified to vars= parameter;
%end;

%if &error %then %goto error;

%if not %length(&dsout) %then %let dsout=%scan(&dsin,1,%str(%());

%if not %length(&lookahead) %then %let lookahead=1;



            /*--------------------------------------*
                   Start processing the data
             *--------------------------------------*/

data _look;
  retain _seq 0;
  set &dsin;
  by &bygroup;
  if first.%scan(&bygroup,-1,%str( )) then _seq=0;
  _seq=_seq+1;
run;

%do i=1 %to &lookahead;
  data _look&i;
    set _look(keep=_seq &bygroup &vars);
    _seq=_seq-&i;
    rename
    %do j=1 %to %words(&vars);
      %scan(&vars,&j,%str( ))=%scan(&vars,&j,%str( ))&i
    %end;
    ;
  run;  
%end;

data &dsout;
  merge _look(in=_look)
  %do i=1 %to &lookahead;
        _look&i
  %end;
        ;
  by &bygroup _seq;
  if _look;
  drop _seq;
run;



            /*--------------------------------------*
                         Tidy up and exit
             *--------------------------------------*/

proc datasets nolist;
  delete _look
  %do i=1 %to &lookahead;
    _look&i
  %end;
  ;
run;
quit;


%goto skip;
%error:
%put ERROR: (lookahead) Leaving macro due to error(s) listed.;
%skip:
%mend;