Dynamically switch input files in a datastep.

Often files are organised into individual files per week, month, year, or goegraphic location. Rather than using multiple datasteps to read from multiple versions, it is better to use a single datastep and switch to a new input file during processing. This is possible using the FILEVAR and END options on the INFILE statement.

FILEVAR allows you to specify a variable whose change in value signifies that the current input file is closed, and a new one opened.

END allows you to specify a variable which will be set to TRUE when the end of the file is met.

The combination of these two variables allows us to detect when we get to the end of the current input file, and switch to a new one. A few things to be careful of here ;

Make sure the variable used by FILEVAR is of sufficient length to hold the full filename.
Appoint an initial filename for the first value.
Make sure the END variable is tested upon every iteration.

An example is shown below ;

%Macro PayW(StartWk,EndWk);
filename infilly "LIVE.ABINITIO.FEED.YRWK&StartWk" disp=shr;
%let Firstwk=%eval(&startwk+200000);
Data TestCall(Drop=infilly weekptr) / View=TestCall;
   Length WEEK £ 6 Area 8 DEPT £ 3 infilly £ 80;
   Retain WeekPtr &startwk week "&Firstwk"
          infilly "LIVE.ABINITIO.FEED.YRWK&startWk";
   Infile infilly filevar=infilly End=EOF;
   Input @1 Area 8.
         @9 Dept £3.
         @15 Sales pd5.;
   If Area="XOM" Then Output;
   If EOF And WeekPtr < &EndWk Then Do;
      WeekPtr + 1;
      If WeekPtr-(int(weekptr/100)*100)=53 Then WeekPtr + 48;
      SubStr(Infilly,21,4)=Put(weekptr,z4.);
      Week=Put(weekptr+200000,6.);
   End;
Run;
Filename infilly;
%Mend;
%PayW(&Stw,&ndW);

Here the macro defined has start and end week input parameters.
The initial filename is assigned, and a macro variable defined which gives us the full date (only part of the date is in the filename in this case).
The filename used on the FILEVAR option is called infilly. The initial value of this is set in a retain statement.
The END option is used against variable name EOF.
After we've used a subsetting IF to reduce the output, we're testing for the end of file (when EOF will be TRUE), and subsequently updating the pointers and part of the filename which holds the week.