uniform(_N_)

james_peacock — Thu, 01 Nov 2018 14:04:29 GMT

Hi,

I'm new to SAS and I can't seem to find out what the _N_ represents in the following bit of code:

uniform(_N_)

It doesn't seem to have been defined anywhere within the code.

Thanks

Re: uniform(_N_)

RW9 — Thu, 01 Nov 2018 14:16:52 GMT

In a datastep, and with some procedures and such like, hidden variables get created at certain points. All documented in the user manual. _n_ is one of those variables and represent logic position of row in the dataset, so first row is 1, second 2 etc.

Re: uniform(_N_)

james_peacock — Thu, 01 Nov 2018 14:18:43 GMT

Thank you very much!!

Re: uniform(_N_)

FreelanceReinh — Thu, 01 Nov 2018 16:50:48 GMT

Hi @james_peacock and welcome to the SAS Support Communities!

In many DATA steps the value of automatic variable _N_ coincides with some observation number. But there are many exceptions to this rule of thumb because in fact _N_ doesn't enumerate observations, but iterations of the DATA step (see Overview of DATA Step Processing and for automatic variables in general: Automatic Variables).

Here are some examples (note that PUT statements write to the SAS log by default):

/* Read 19 observations: _N_ and observation number coincide */

data _null_;
set sashelp.class;
put _n_=;
run;

/* Make 20th DATA step iteration visible */

data _null_;
put _n_=;
set sashelp.class;
run;

/* Read and write 19 obs. within a single iteration of the DATA step: _N_=1 */

data test;
do until(0); /* (no infinite loop thanks to SET statement) */
  set sashelp.class;
  output;
  put _n_=;
end;
run;

/* Create dataset from data lines: _N_=1, 2, 3 */

data a;
input x;
put _n_=;
cards;
10
20
30
;

/* Create same dataset from same data lines, but within
   one iteration of the DATA step: _N_=1 */

data a;
input x;
output;
input x;
output;
input x;
output;
put _n_=;
cards;
10
20
30
;

/* No datasets involved, still _N_=1 */

data _null_;
put _n_=;
run;

/* _N_ can be "mis"used for other purposes (here: as an index variable 
   of a DO loop that is dropped automatically). It's not read-only. */

data squares;
do _n_=1 to 5;
  square=_n_**2;
  output;
end;
run;

If your example refers to the UNIFORM function, using uniform(_N_) is possibly questionable or misleading (depending on the context) not only because this function is deprecated, but because multiple calls of the function with varying values of _N_ in the same DATA step always use the same seed value.

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