topic Re: is PDV apply while writing the data _null_? in SAS Programming

is PDV apply while writing the data _null_?

thanikondharish — Mon, 02 May 2022 10:32:27 GMT

/*example1*/

data ex1;

set sashelp.class ;

run;

Note: (exmaple1) In above step backend PDV concept will be executed.

/*example2*/

data _null_;

set sashelp.class;

put _all_;

run;

my question is like in above step (example2) is backend PDV concept will be applied .
i think not applied . Could you please guide me?

Re: is PDV apply while writing the data _null_?

andreas_lds — Mon, 02 May 2022 10:46:19 GMT

Why do you think that the pdv-concept is not applied in a data _null_ step?

Re: is PDV apply while writing the data _null_?

ChrisHemedinger — Mon, 02 May 2022 11:42:11 GMT

The PDV (program data vector) is present/used in DATA step even when you aren't writing an output data set. To see it in action and get more insight into how it works, use the DATA step debugger (tutorial here). It's a great learning tool.

Re: is PDV apply while writing the data _null_?

thanikondharish — Mon, 02 May 2022 11:48:24 GMT

No.of iterations won't be formed in new dataset (just an idea)

Can you guide exact point?

Re: is PDV apply while writing the data _null_?

Quentin — Mon, 02 May 2022 13:05:13 GMT

As Chris wrote, the PDV is a logical component of the DATA step itself. If you are writing a DATA step, it has a PDV.

In your sample code code when you use the PUT statement, you are writing values from the PDV to the log. I added a BY statement for fun, and here is the log:

1    data _null_ ;
2      set sashelp.class ;
3      by name ;
4      put _all_;
5    run ;

Name=Alfred Sex=M Age=14 Height=69 Weight=112.5 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=1
Name=Alice Sex=F Age=13 Height=56.5 Weight=84 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=2
Name=Barbara Sex=F Age=13 Height=65.3 Weight=98 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=3
Name=Carol Sex=F Age=14 Height=62.8 Weight=102.5 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=4
Name=Henry Sex=M Age=14 Height=63.5 Weight=102.5 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=5
Name=James Sex=M Age=12 Height=57.3 Weight=83 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=6
Name=Jane Sex=F Age=12 Height=59.8 Weight=84.5 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=7
Name=Janet Sex=F Age=15 Height=62.5 Weight=112.5 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=8
Name=Jeffrey Sex=M Age=13 Height=62.5 Weight=84 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=9
Name=John Sex=M Age=12 Height=59 Weight=99.5 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=10
Name=Joyce Sex=F Age=11 Height=51.3 Weight=50.5 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=11
Name=Judy Sex=F Age=14 Height=64.3 Weight=90 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=12
Name=Louise Sex=F Age=12 Height=56.3 Weight=77 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=13
Name=Mary Sex=F Age=15 Height=66.5 Weight=112 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=14
Name=Philip Sex=M Age=16 Height=72 Weight=150 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=15
Name=Robert Sex=M Age=12 Height=64.8 Weight=128 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=16
Name=Ronald Sex=M Age=15 Height=67 Weight=133 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=17
Name=Thomas Sex=M Age=11 Height=57.5 Weight=85 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=18
Name=William Sex=M Age=15 Height=66.5 Weight=112 FIRST.Name=1 LAST.Name=1 _ERROR_=0 _N_=19
NOTE: There were 19 observations read from the data set SASHELP.CLASS.

You can see there are 9 variables in the PDV. Five are read from sashelp.class. Four are automatic variables created by SAS.

The data step works by creating the PDV, reading values into the PDV, changing values in the PDV, then writing values from the PDV. There is always a PDV.

As can be seen above, the counter of DATA step iterations, _N_, is always present in the PDV. The automatic variables _N_ _ERROR_ FIRST.Name LAST.Name are not written to an output dataset, but they exist in the PDV.

+1 For Chris's recommendation to play around with the DATA step debugger. Learning about the PDV is critical to understanding the DATA step, which is critical to understanding SAS.

Re: is PDV apply while writing the data _null_?

mkeintz — Mon, 02 May 2022 13:05:52 GMT

The PDV is constructed by the SAS compiler - in advance of actual data processing. Before any data processing occurs, the compiler makes provision for each variable encountered in a SET, MERGE, INPUT and other data-retrieval statements as well as assignment statements that will calculate values for new variables.

And remember, a DATA _NULL_ step does process data, so it would need the PDV to keep variables organized during that process.