topic Re: Question: _n_ not change? in SAS Software for Learning Community

Question: _n_ not change?

jz127323 — Thu, 07 Oct 2021 19:26:23 GMT

Hi, I have the following data step. But I find that the ID (_n_) is always equal to 1. 
Should it be 1, 2, ...10?
Thanks!



data Easyway;
	do Group='Placebo', 'Active';

		do Subj=1 to 5;
		    ID = _n_;
			input Score @; 
			output; 			
		end;
	end;
	datalines;
250 222 230 210 199
166 183 123 129 234
;
run;

Re: Question: _n_ not change?

Tom — Thu, 07 Oct 2021 19:31:33 GMT

Why would _N_ change? You didn't do anything to make it change. Like assigning it a value. Or starting the next iteration of the data step, which is what normally makes _N_ increment.

What is the difference between SUBJ and ID in your dataset?

If you want ID to be 1 when GROUP is 'Placebo' and 2 then it is 'Active' then set its value inside the outer DO loop.

do Group='Placebo', 'Active';
   ID+1;
   do SUBJ=1 to 5;
   ...

Re: Question: _n_ not change?

ballardw — Thu, 07 Oct 2021 19:31:51 GMT

Incomplete data step will not run, no DATA at the top

The _n_ variable indicates how many times the data step executes, i.e. hits the bottom of the code and starts over.

Where you are using _n_ is in a loop. So the value that it has at the start of the DO loop stays the same until the loop finishes and then hits the second end.

If I understand what you want try this:

data example;
   do Group='Placebo', 'Active';
		do Subj=1 to 5;
		    ID+1;
			input Score @; 
			output; 			
		end;
	end;
datalines;
250 222 230 210 199
166 183 123 129 234
;
run;

The statement ID+1 implies the ID value is retained across iterations of the data step and each time the statement is encountered in the loop 1 is added.