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Date:         Tue, 20 Jul 2010 16:02:11 -0400
Reply-To:     Ian Whitlock <iw1sas@GMAIL.COM>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         Ian Whitlock <iw1sas@GMAIL.COM>
Subject:      Is LEAVE "non-structured"?
Comments: cc: "Data _null_," <iebupdte@gmail.com>,
          Robert Rosofsky <Robert.Rosofsky@VERIZON.NET>
Content-Type: text/plain; charset=ISO-8859-1

Data _null_ and Robert,

The argument is an old one from the 1960's.

First let's separate languages. You cannot argue that I can use GOTO in SAS because the computer at some point executes a jump or goto. That would be similar to saying SAS should be written as strings of 0 and 1 because that is what is ultimately implemented. The whole point of a higher level language is to be free of the method of implementation. So the question becomes should a high level language have an explicit GOTO.

The disadvantages: 1) Code quickly becomes unreadable with arbitrary GOTOs. 2) Proving code correct becomes much harder with explicit GOTOs.

The advantages: A well placed GOTO can make the code more readable. Example, escaping from the middle of a nested sequence of loops.

The vote of history appears to be that most languages have an explicit GOTO and some of those that did not start with one soon added a GOTO.

The biggest problem with unreadable GOTOs is their uncontrolled nature. If one only allows transfer to a later point in the code the unreadable problem largely disappears. LEAVE enforces that policy.

So what is the difference between blah, blah, goto and blah, blah, leave. In the second case the reader immediately knows what has happened. In the first case the reader must find the target location to confirm what the reader of the second case already knew. If you are the only reader of your code and you follow a good policy in the use of GOTOs then it doesn't matter much.

Personally I would not write GOTO when LEAVE accomplishes the same thing. However, it does not bother me much to write

%goto mexit ;

in macros although the twinges have gotten stronger since %RETURN has been introduced.

In short there are good reasons to avoid GOTO's and good reasons to allow one once in while. Hence, in the current state of the art of writing computer languages, the language should not prevent a GOTO, but the programmer should most of the time not use them.

Now what about LEAVE versus WHILE and UNTIL? WHILE guarantees exit at the top of the loop and UNTIL guarantees it at the bottom. Code is somewhat easier to read if you know exactly where the exit is and under what conditions it will happen. On the other hand, a simple LEAVE in the middle of a loop may be clearer than setting a flag and using an IF/ELSE construction to make the code exit at the bottom of a loop. A second LEAVE makes even more true.

When one starts having loops involving many lines and many LEAVEs then it is time to leave the code and work on a better design.

Rules are aids to writing readable programs, but the programmer should understand why the rule, so that he/she can know when it is wise to break the rule.

Ian Whitlock ===============

Date: Mon, 19 Jul 2010 09:09:32 -0500 From: "Data _null_;" <iebupdte@GMAIL.COM> Subject: Is LEAVE "non-structured"? Content-Type: text/plain; charset=ISO-8859-1

In a reply from Robert Rosofsky in the "Combining iterative DO and UNTIL condition" where I suggested the LEAVE statement he indicated

[quote]The use of the LEAVE command feels kind of "non-structured," but I suppose I can live with that.[/quote]

I use LEAVE often and sometimes prefer it to WHILE/UNTIL. I used GOTO before we had LEAVE and still use GOTO when "I think I need to". Aren't all the programming constructs are just dressed up GOTOs.

So is LEAVE non-structured?


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