Date:         Tue, 19 Feb 2008 08:13:26 -0500
Reply-To:     Arthur Tabachneck <art297@NETSCAPE.NET>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         Arthur Tabachneck <art297@NETSCAPE.NET>
Subject:      Why does retain work faster conditionally?
Content-Type: text/plain; charset=ISO-8859-1

One of our most respected list members wrote me off-line, asking why in
the world I would have suggested wrapping a retain statement within a
condition.

That is, given the following data:

data have;
    input lname$ fname$;
    do i=1 to 1000000;output;end;
    cards;
    lname1 fname1
    lname2 fname2
;

why write:

data want;
  if _n_ eq 1 then do;
    retain fname;
  end;
  set have;
run;

instead of:
data want;
  retain fname;
  set a;
run;

I know why I provided the solution, because it had better performance, but
I could sure use some feedback explaining why that would be so.

I initially wrote it correctly and, upon seeing that it worked slower than
Jiann's SQL solution, tried to see if I could bypass reading the data
(i.e., when _n_ eq 0).

After I soon realized that wouldn't be possible, I ran the step as
presented.

Someone please explain to me why:

60   data want;
61     if _n_ eq 1 then do;
62       retain fname;
63     end;
64     set a;
65   run;

NOTE: There were 2000000 observations read from the data set WORK.A.
NOTE: The data set WORK.WANT has 2000000 observations and 3 variables.
NOTE: DATA statement used (Total process time):
      real time           1.12 seconds
      cpu time            1.12 seconds

runs almost 50% faster than:
56   data want;
57     retain fname;
58     set a;
59   run;

NOTE: There were 2000000 observations read from the data set WORK.A.
NOTE: The data set WORK.WANT has 2000000 observations and 3 variables.
NOTE: DATA statement used (Total process time):
      real time           1.43 seconds
      cpu time            1.43 seconds

I ran the tests on a 4-processor Window's 2003 system with 12 gig of ram
and SAS 9.1.3.  It was during a holiday, thus I was the only one using the
computer and I re-ran the tests 3 times with the same results.

Art
--------
On Mon, 18 Feb 2008 23:21:23 -0500, Arthur Tabachneck
<art297@NETSCAPE.NET> wrote:

>Miguel,
>
>As Jiann indicated, you can do what you want with proc sql.  However, you
>can also accomplish the same thing in a data step.  For example,
>
>data have;
>    input lname$ fname$;
>    do i=1 to 1000000;output;end;
>    cards;
>    lname1 fname1
>    lname2 fname2
>    ;
>
>data want;
>  if _n_ eq 1 then do;
>    retain fname;
>  end;
>  set have;
>run;
>
>HTH,
>Art
>---------
>On Tue, 19 Feb 2008 02:55:04 +0000, Miguel de la Hoz <miguel_hoz@YAHOO.ES>
>wrote:
>
>>I am starting my problem with the following disposal of my dataset:
>
>#  variable
>1  lname
>2  fname
>
>I am trying to export it to excel but it is keeping that order. I would
>like to be able to write
>
># variable
>1 fname
>2 lname
>
>This is only an example my dataset contains around 20 fields.
>
>Thanks.
>
>MDH.
>
>
>
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