Date:         Thu, 25 Jul 2002 15:24:30 -0600
Reply-To:     Kenneth Moody <KennethMoody@FIRSTHEALTH.COM>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         Kenneth Moody <KennethMoody@FIRSTHEALTH.COM>
Subject:      Re: Double Assignment
Content-Type: text/plain; charset=us-ascii

Jess,
Evaluation of expressions is covered in the SAS Language Reference:
Concepts manual in Chapter 12.

You've asked about a special case, because var3 is surrounded by two
comparison operations; it's evaluated as:

  var1 = (var2=var3) and (var3 = 7);

In your case var2 is missing, so the evaluation is (.=6) and (6=7),
which is 0 and 0, so var1 = 0;

The left most equal sign is an assignment operator, all of the others
are comparison operators.  The special case of a quantity surrounded by
two comparison operators is what allows us to write expressions like
'1jan2001'd le date_var le '31dec2001'd to test whether a quantity is
within a range.


Ken Moody
First Health, Metrics Department
Voice: 916-374-3924
EMail: KennethMoody@firsthealth.com

>>> "Balint, Jess" <JBalint@ALLDATA.NET> 07/25/02 01:49PM >>>
Actually I was talking about:

var1=var2=var3=7;

Where does SAS decide to put the parenthesis?

Hence:

data _NULL_;
        var3 = 6;
        var1=var2=var3=7;
        put var1 var2 var3;
run;

If or not var3 = 7, var2 is missing and var1 is 0. How is it
evaluated?

Why didn't they use the good old '?:' conditional operator?

-----Original Message-----
From: Ian Whitlock [mailto:WHITLOI1@WESTAT.com]
Sent: Thursday, July 25, 2002 4:53 PM
To: 'Balint, Jess'; SAS-L@LISTSERV.UGA.EDU
Subject: RE: Double Assignment


Jess,

   var2=var3=7

is short for

   (var2 = var3) and (var3 = 7)

I see no reason why VAR2 should always be missing, but if it is then
the
result is 0 because a logical consequence (transitivity of equality) of
the
above expression is that

   var2 = 7

and this is never true when VAR2 is missing.

Never trust what a programmer, or anyone else, tells you.  Test it,
decide
for yourself, and be prepared to revise your opinion when more data is
available to you.  Of course even if you find that every example you
consider leads to the truth of the statement you might wonder if SAS
would
give the same results tomorrow.  If that is true it is time to take up
another language or occupation. (However, note there are exceptions
which
may be allowed.  Remember the result of some code can depend on the
data,
the operating system, the environment (both the out of disk space
variety
and magnets) as well as the language.)

However you must also understand things like (run under winows 98 SAS
8.2)

1602  data _null_ ;
1603     x = 7 ;
1604     y = 7 + 4.44e-16 ;
1605     z = x = y ;
1606     put z = ;
1607  run ;

z=1

The search for truth will always depend on what facts you find and
your
theoretical model of how to understand those facts.

IanWhitlock@westat.com

-----Original Message-----
From: Balint, Jess [mailto:JBalint@ALLDATA.NET]
Sent: Thursday, July 25, 2002 3:48 PM
To: SAS-L@LISTSERV.UGA.EDU
Subject: Re: Double Assignment


How exactly would that work out?

var2 is always missing?!?

-----Original Message-----
From: Sigurd Hermansen [mailto:HERMANS1@WESTAT.COM]
Sent: Thursday, July 25, 2002 3:41 PM
To: SAS-L@LISTSERV.UGA.EDU
Subject: Re: Double Assignment


I would not trust a programmer who leaves out the parentheses. What
rules of
operator precedence apply in multiple assignments across the same
operator?

Try to explain how SAS will evaluate
                         var1=var2=var3=7;
  to someone other than a committed (or soon to be) SAS programmer!

Sig
-----Original Message-----
From: Jack Hamilton [mailto:JackHamilton@FIRSTHEALTH.COM]
Sent: Thursday, July 25, 2002 3:22 PM
To: SAS-L@LISTSERV.UGA.EDU
Subject: Re: Double Assignment


Do you mean you have a hard time trusting this, i.e., that it will
produce the answer of 0 or 1 correctly, or that you have a hard time
believing that that's what the statement means?

Why don't you trust or believe it?  Has something happened to make you
think it doesn't work that way?

Will it help if you think of it as

   var = (var1 = 7)

?


--
JackHamilton@FirstHealth.com
Manager, Technical Development
METRICS Department, First Health
West Sacramento, California USA


>>> "Balint, Jess" <JBalint@ALLDATA.NET> 07/25/2002 11:54 AM >>>
Hello all. I have seen people here write code like "var2 = var1 = 7".
From
what they say var2 = 1 if var1 = 7 and var2 = 0 otherwise. I have a
hard
time trusting this. Is it correct though? Thanks.

Jess