Avi - SAS is full of surprises! ;-)

This retain action happens because it's the default action of the data step. Variables that are automatically retained are those that are reloaded with each iteration, so the retain action isn't usually noticed - as you indicate. It's a useful feature for a variety of reasons, but the fact that it is default on some and not all variables might be surprising.

The RETAIN statement is redundant for: - Variables defined by SET, MERGE, MODIFY or UPDATE statements, - Variables defined by statement options in SET, MERGE, MODIFY, UPDATE, FILE and INFILE statements, - Sum statements (i.e. "I+1;"), - Initialized array statement variables

Search the list/SUGI papers on the DOW Loop, the SAS Supervisor, and the PDV to get more information on how the data step works. Start by reading the onlinedoc sections on the data step: http://support.sas.com/onlinedoc/913/getDoc/en/lrcon.hlp/a000985872.htm

There's a little table that resides alongside the PDV called the ITMV (Initialize to Missing Vector) - this flags the retain action on the variables. It's a one-way process - variables can be set to "retain" but not set to "don't retain". There's another 1-1 table with the PDV called the DKT - (Drop Keep Table) - it has a similar sort of purpose.

If you _really_ want to clear all variables brought in with a set statement then something like this could do the trick:

data sasadata(drop=:_); if 0 then set sasdata; array nums _numeric_; array chars $ _char_; do _i = 1 to dim(nums); nums(_i)=.; end; do _j = 1 to dim(chars); chars(_j)=''; end; *program statements; (conditionally) set sasdata; *more program statements; run;

This reads the structure of the incoming data set (but no data because 0 is false) to build the PDV correctly and give the arrays the variables to reset. The arrays reset the number vars to missing and the character vars to blank. Then the data is conditionally read in.

Paul Choate DDS Data Extraction (916) 654-2160

-----Original Message----- From: SAS(r) Discussion [mailto:SAS-L@LISTSERV.UGA.EDU] On Behalf Of avi Sent: Tuesday, October 03, 2006 3:47 PM To: SAS-L@LISTSERV.UGA.EDU Subject: Surprising retain feature

Hello,

Though an quite experienced user of SAS, i was quite surprised to learn that in some cases, observations retain their values from one obs to another, without specifying the RETAIN statement ("When variables are read with a SET, MERGE, or UPDATE , The variables retain their values until new values become available").

Could someone explain exactly what and why it happens and the way to overcome the problem thaht has caused some disturbing results in one of my applications?

Thaks a lot Avi