Date:         Fri, 13 Mar 2009 16:36:09 -0500
Reply-To:     Tim Kynerd <tkynerd@ECD.ORG>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         Tim Kynerd <tkynerd@ECD.ORG>
Subject:      Re: SUM statement paradox (bug)?
Content-Type: text/plain; charset="us-ascii"

Hi Paul,

To echo the other folks, here's the behavior I'm seeing in SAS 9.1.3:

When I run your code, I get what you get: var=. var=7 .
When I comment out the SET statement in the second DATA step, I get: var=0 . As I would expect, the second DATA step only executes once because it has no input.

I knew about the implied RETAIN with the Sum statement, but I didn't know this:

----
Tip:    The variable is automatically set to 0 before SAS reads the first observation. The variable's value is retained from one iteration to the next, as if it had appeared in a RETAIN statement.
Tip:    To initialize a sum variable to a value other than 0, include it in a RETAIN statement with an initial value.
----

As to why VAR is initially missing in the first step:

----
It is redundant to name any of these items in a RETAIN statement, because their values are automatically retained from one iteration of the DATA step to the next:

    *

      variables that are read with a SET, MERGE, MODIFY or UPDATE statement
    *

      a variable whose value is assigned in a sum statement
    *

      the automatic variables _N_, _ERROR_, _I_, _CMD_, and _MSG_
    *

      variables that are created by the END= or IN= option in the SET, MERGE, MODIFY, or UPDATE statement or by options that create variables in the FILE and INFILE statements
    *

      data elements that are specified in a temporary array
    *

      array elements that are initialized in the ARRAY statement
    *

      elements of an array that have assigned initial values to any or all of the elements on the ARRAY statement.

You can, however, use a RETAIN statement to assign an initial value to any of the previous items, with the exception of _N_ and _ERROR_.
----

The fact that SAS emphasizes the option of using a RETAIN statement to assign an initial value makes it seem natural to me, if not strictly *logical*, that variables in these cases are initialized to missing, as they apparently are.

Best,
Tim


Tim Kynerd
Computer Programmer/Analyst
ECD/HOPE
4 Old River Place, Suite A
Jackson, MS 39202
P: (601) 944-9308
F: (601) 944-0808
tkynerd@ecd.org

-----Original Message-----
From: SAS(r) Discussion [mailto:SAS-L@LISTSERV.UGA.EDU] On Behalf Of Paul Dorfman
Sent: Friday, March 13, 2009 1:16 PM
To: SAS-L@LISTSERV.UGA.EDU
Subject: SUM statement paradox (bug)?

Y'all,

For those inclined to "ponder over many a quaint and curious volume of
forgotten lore", consider:

data huh ;
   retain var 7 ;
run ;

data _null_ ;
   put var = @ ;
   var + 1 ;
   set huh ;
run ;

Paradox #1. I expect the second step to print var=0 var=7. It prints var=.
var=7 instead, as if the SUM statement were not present.

Paradox #2. If VAR is not in HUH, or is dropped from it, or SET is simply
commented out, the step happily prints the expected var=0 var=7.

It appears that the mere act of the compiler finding VAR in the descriptor
of a data set named in a syntactically valid context changes the implicit
sum-statement compiler action into RETAIN VAR . instead of the expected
RETAIN VAR 0 (or, perhaps more likely, just abstains from setting VAR to
zero and leaves it missing).

I discovered it a hard way - a production load failed having detected a
missing value for a NOT NULL column - and stared at the step in giddy
incredulity for quite a while before I found that the reason was the SUM
statement behaving not in the way it is documented.

For nowhere in the SAS documentation this peculiar behavior is mentioned. I
only find the unequivocal assertions "the variable is automatically set to
0 before SAS reads the first observation" and "The sum statement is
equivalent to using the SUM function and the RETAIN statement, as shown
here: retain variable 0; variable=sum(variable,expression);", for some
reason listed under the "Tip" rubrics, as if the fundamental property of
the SUM statement seared in the mind of any semicolon-worthy SAS programmer
is but a useful side effect.

Since both directly contradict the experimental results capable of biting
an unwary, I hereby declare it a bug.

Kind regards
------------
Paul Dorfman
Jax, FL
------------

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