Assuming you're new to SAS, one key point is that the SAS DATA step is a loop. So you don't have to write a loop to loop through records in a dataset. A simple data step reads a one record from a dataset, processes the record, outputs it and then loops to read the next record.
Sharing data as code makes it easier for people to help you. You have data:
data have ;
input dates vins $5.;
cards ;
1880 4V6HJ
1880 4V6HJ
1880 6C8KI
1881 4V6HJ
1884 6C8KI
;
run ;
You can create a counter with code like:
data want ;
set have ;
by dates vins ;
if first.vins then counter++1 ;
put (dates vins first.dates first.vins counter)(=) ;
run ;
Which will return:
20 data want ;
21 set have ;
22 by dates vins ;
23 if first.vins then counter++1 ;
24 put (dates vins first.dates first.vins counter)(=) ;
25 run ;
dates=1880 vins=4V6HJ FIRST.dates=1 FIRST.vins=1 counter=1
dates=1880 vins=4V6HJ FIRST.dates=0 FIRST.vins=0 counter=1
dates=1880 vins=6C8KI FIRST.dates=0 FIRST.vins=1 counter=2
dates=1881 vins=4V6HJ FIRST.dates=1 FIRST.vins=1 counter=3
dates=1884 vins=6C8KI FIRST.dates=1 FIRST.vins=1 counter=4
NOTE: There were 5 observations read from the data set WORK.HAVE.
NOTE: The data set WORK.WANT has 5 observations and 3 variables
That simple data step makes use of a few novel SAS constructs, which are needed because the data step processes records one record at a time.
The first is BY-group processing. The statement BY Dates Vins; tells SAS to create temporary boolean variables first.dates and first.vins which will be set to 1 (true) when the value changes.
The statement counter++1; is an example of using the SUM statement to create an accumulator.
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