Assign ID to pairs of consecutive rows

[STLT]

Hi,

I have a dataset of individuals already sorted on a value. I would like to assign a pairID to the first two individuals, then the next pair, and so on. If the file contains an odd number of individuals, the last three are grouped to form a triplet. I hope that makes sense and would appreciate any tips. 

Thanks!

HAVE		WANT
ID		ID	pairID
1		1	1
2		2	1
3		3	2
4		4	2
5		5	3
6		6	3
7		7	4
8		8	4
9		9	5
10		10	5
11		11	5

[Reeza · Accepted solution]

Playing around with some basic math this is relatively straightforward. 

Functions: 

MOD() 

Use NOBS option on SET statement to get the number of observations. 

Use RETAIN 

Data want;

set have NOBS=num end =eof;

RETAIN count;

If mod(_n_, 2) ne 0 and not eof then count+1;

run;

[Haikuo]

Of course I would choose to use @Reeza's solution myself, but FWIW, here is another approach outrageously not using MOD() ,

data want;
do _error_=1 to 2;
set sashelp.class end=last;
pairID=ifn(last and _error_=1,_n_-1,_n_);
output;
end;
run;

[STLT]

Thank you, both, for taking time to resolve this for me. I can't believe how few lines of codes are needed. I understand the logic, but am just not advanced enough to come up with it myself. Thanks again for the help!