Xref: utzoo misc.misc:5957 comp.misc:5959
Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uflorida!indri!ames!hc!lanl!jlg
From: jlg@lanl.gov (Jim Giles)
Newsgroups: misc.misc,comp.misc
Subject: Re: The "evil" GOTO (Was: 25 Years of BASIC)
Message-ID: <13205@lanl.gov>
Date: 5 May 89 21:02:15 GMT
References: <2854@cps3xx.UUCP>
Organization: Los Alamos National Laboratory
Lines: 59

From article <2854@cps3xx.UUCP>, by rang@cpsin3.cps.msu.edu (Anton Rang):
>>	 printf ("Enter your sex: ");
>>	 for (;;) {
>>	    gets (sex);
>>	    if (sex == "m" || sex == "f") break;
>>	    printf ("<m> or <f> only: ");
>>	 }
> This is ugly; it essentially transforms the code
> [...] 
> into
> 
> 	LABEL: get input
> 	       if (good input)
> 		 goto LABEL2
> 	       print message
> 	       go back to label
> 	LABEL2:

You are exactly right.  I didn't say the feature was elegant, I said that
"loop-and-a-half" problems were the reason that "break" was introduced
into looping constructs (which occured in languages long before C was
invented).  The reason for "break" in "case" constructs only dmr can
answer 8-).
 
> [...]                              Because the place the break goes to
> isn't explicit; you have to start matching up loops.  If you have lots
> of nesting, this is quite a mess.
>   Named loops in Ada are intended to alleviate this problem, and if
> used properly seem to do a decent job.

I like the solution of named loops too.  But Ada puts all but the first
occurrance of the the label into the code rather than on the left margin.
The advantage of GOTOs (if any) was that you could easily find the target
of the branch by looking for the label.  Consider the example on page 5-8
of the Ada standard:

      MAIN_CYCLE: loop
                   --  initial statements
                   exit MAIN_CYCLE when FOUND;
                   --  final statements
                   end loop MAIN_CYCLE;

If the "initial" and "final" statement sequences were long and complicated
the "exit" and "end loop" could be difficult to identify.  (Even indentation
only helps a little if the code crosses page boundaries.)  In the Nemesis
programming language the above loop would be rendered as:

      MAIN_CYCLE: do
                  --  initial statements
      MAIN_CYCLE: exit if (FOUND)
                  --  final statements
      MAIN_CYCLE: end do

Here, no matter how long and complicated the statement sequences are, all
the controls for the "MAIN_CYCLE" loop are easily identified by scanning
the left margin of the code.  Note that the identifier "MAIN_CYCLE" is
the _name_ of a loop and _not_ a statement label (you can't GOTO MAIN_CYCLE).
Loop names _must_ appear on all control statements for the loop.  Unnamed
loops are allowed and must _not_ have a name on any control statement.