Date:         Wed, 5 Apr 2006 12:08:59 -0700
Reply-To:     "Terjeson, Mark (IM&R)" <Mterjeson@RUSSELL.COM>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         "Terjeson, Mark (IM&R)" <Mterjeson@RUSSELL.COM>
Subject:      Re: Identifying the recent history for transaction,
              Problem Retain lag ????
Comments: To: M <mtamboli@GMAIL.COM>
Content-Type: text/plain; charset="US-ASCII"

Hi M,

The common misinterpretation of LAG()
is that it is always pointing to the
previous observation.  While that is
the desired objective many times, the
way the LAG() function works actually
allows much more flexibility and power.

Interpreting the LAG() function as always
pointing to the previous observation is
true if the LAG() is *not* inside *any*
conditional statements.  If you have
a LAG() for the same desired value inside
a conditional statement and you are
thinking LAG() is always the last obs then
you get out of sync very quickly.

The LAG() is actually the previous value
encountered by the LAG() function.  So if
conditional statements are incurred you
have a queue of only selected values to
retreive.

The easiest way to handle this is to place
a separate assignment outside any conditional
code so that the retreivals from LAG() really
are from the previous observation.  e.g. after
the SET statement (or whatever brings obs in)
then do something like this:
    the_lagged_value = LAG(myvar);
Then do your conditional stuff and use the
    the_lagged_value variable to your hearts
content.  This way you can scatter your use
of the_lagged_value throughout your conditional
code as often as you want and always be in sync.



Hope this is helpful.


Mark Terjeson
Senior Programmer Analyst, IM&R
Russell Investment Group


Russell
Global Leaders in Multi-Manager Investing










-----Original Message-----
From: SAS(r) Discussion [mailto:SAS-L@LISTSERV.UGA.EDU] On Behalf Of M
Sent: Wednesday, April 05, 2006 11:48 AM
To: SAS-L@LISTSERV.UGA.EDU
Subject: Identifying the recent history for transaction, Problem Retain
lag ????


Hello,

I am not able to figure out what the problem is.
Here is what i am trying to do
Based on the history bit  (hist_bit) of the current record do some
action. if hist_bit eq 1 then  i am trying to assign the previous
product information to the  current  record.

Here is an example

Input (work.check1)

Name       Prod_Name     hist_bit
Tom        Certificate          0
Tom                                1
Tom                                1
Tom                                1

Desired_Output

Name       Prod_Name  hist_bit
Tom        Certificate      0
Tom        Certificate      1
Tom        Certificate      1
Tom        Certificate      1

However here is what i am getting based on the code below. Check is a
flag set to determine what condition is being executed. I would
appreciate if someone could help me in figuring out where the problem
lies.

Thanks,

Name       Prod_Name     hist_bit    check       flag    prod_hist
Tom        Certificate             0         5              0
Tom                                   1         3               1
Tom                                   1         3               1
Tom                                   1         3               1

/* Code begines here */

data temp;
   set check1;
   BY NAME;
   format prod_hist $CHAR50.;
   retain prod_hist  flag;

   if (FIRST.NAME)then do;
                                  prod_hist = '         ';
                                  flag = 0;
                                 check = 5;
                                 end;

   if NOT(FIRST.NAME)then

                                 do;
                                 if (hist_bit eq 0) then
                                 do;
                 prod_hist = '          ';
                         flag = 0;
                 check = 6;
                 end;

                                 if (hist_bit eq 1) then
                                 do;
                            if (PROD_NAME eq ' ') AND (flag  eq  0) then
                                    do;
                    prod_hist = lag (PROD_NAME);
                    check = 1;
                            PROD_NAME = prod_hist;
                    flag = 1;
                    end;

                            if (PROD_NAME ne ' ') AND (flag  eq  0) then
                                    do;
                    prod_hist = PROD_NAME;
                            flag = 1;
                    check = 2;
                    end;

                    if (PROD_NAME eq ' ') AND (flag  eq  1) then
                                    do;
                    PROD_NAME = prod_hist;
                    flag = 1;
check = 3;
                   end;

                   if (PROD_NAME ne ' ') AND (flag  eq  1) then
                                   do;
                   prod_hist = PROD_NAME;
                           flag = 1;
                   check = 4;
                   end;
                        end;
                      end;
run;