This is reminiscent of Bob Virgile's contest problems.

The DATA step with the explicit loop will actually begin execution twice, not once.

Try inserting the statement

put _N_=;

between the DO and SET statements. The log will then contain 11 dumps of the automatic variable. The first ten will have the value 1 and the eleventh will have the value 2.

The DATA step here does not terminate until the SET fails to return a new observation. In this case that happens on the first iteration of the DO loop during the second iteration of the DATA step.

This is a technical clarification only; I agree with Greg's suggestion.

On Tue, 31 Oct 2000 13:14:44 -0600, Greg Woolridge <greg.woolridge@TAP.COM> wrote:

>_n_ is the number of times the data step has begun executing, which does >not necessarily corrspond to the observation being read from the data set, >as you have discovered. In the creation of B you have the SET statement >within a DO loop. The data step executes only 1 time and the DO loops as >many times as needed to read all observations. Since the data step >executes only 1 time, _n_ is never incremented. I would suggest using a >counter instead as below: > >data B ; > do until (EOF) ; > set A end=EOF ; > N + 1; > output ; > end ; >run ; > > >Greg M. Woolridge >Manager, Study Programming >TAP Pharmaceutical Products Inc. >e-mail: greg.woolridge@tap.com >phone: 847-582-2332 >fax: 847-582-2403 > > > > Bill Knowlton > <Bill_Knowlton@B To: SAS-L@LISTSERV.UGA.EDU > AXTER.COM> cc: > Sent by: "SAS(r) Subject: SAS/Base: automatic variable _n_ behavior > Discussion" > <SAS-L@LISTSERV. > UGA.EDU> > > > 10/31/00 12:33 > PM > Please respond > to Bill_Knowlton > > > > > >Hi all, > >In the code included below, 'was wondering if anyone can explain why the >automatic variable _n_ doesn't increment for the code generating dataset B >but does for the code generating dataset C. > >What does _n_ represent? I had thought it represented the current >observation number read from the input dataset. Is there some other >automatic variable that holds this value? I've had no luck in finding a >description of _n_ in the SAS help, other than that it is a reserved name. > >Thanks for any insights, > >-Bill > > >data A ; > do i = 1 to 10 ; > output ; > end ; >run ; > >data B ; > do until (EOF) ; > set A end=EOF ; > N = _n_ ; > output ; > end ; >run ; > >data C ; > set A ; > N = _n_ ; > output ; >run ;