Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!swrinde!elroy.jpl.nasa.gov!jato!vsnyder
From: vsnyder@jato.jpl.nasa.gov (Van Snyder)
Newsgroups: comp.lang.fortran
Subject: Re: Fortran 90 status
Message-ID: <1991May21.203019.21452@jato.jpl.nasa.gov>
Date: 21 May 91 20:30:19 GMT
Reply-To: vsnyder@jato.Jpl.Nasa.Gov (Van Snyder)
Distribution: comp
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 55

I tried for several years to get X3J3 to extend CASE to include REAL ranges.
I explained carefully that this was nothing different from IF...ELSE IF ...
ELSE IF ... ELSE ... END IF, but that the part of the predicate repeated in
each ELSE IF was put into the SELECT CASE statement instead.  All I got
by way of explanation for failure to extend CASE was that:
1.  "Real relationals were a mistake in Fortran 77 that we didn't want to
    repeat."
2.  "Real induction variables in DO in Fortran 77 were a mistake that we
    didn't want to repeat."
3.  "Real subscripts ... "
4.  "There will be non-portable round-off problems."
5.  "There WILL be non-portable round-off problems."
Nobody explained any of these.  How can allowing a programmer to write
a clearer, more optimized version of IF ... ELSE IF ... ELSE IF ... ELSE 
block cause any of the above problems?
BTW, the attraction of Arithmetic IF is that it does a little bit of what
extending CASE to REAL ranges would allow.  The alternative to

   IF (e) 10,20,30

SHOULD have been

   SELECT CASE e
   CASE * < 0
      blah blah (statement 10ff above)
   CASE * = 0
      blah blah (statement 20ff above)
   CASE * > 0 ! or DEFAULT
      blah blah (statement 30ff above)
   END CASE

instead of

   IF (e.lt.0) THEN
     blah blah 10
   ELSE IF (e.eq.0) THEN
     blah blah 20
   ELSE
     blah blah 30
   END IF

which requires "e" to be evaluated twice, or the almost as repulsive

   temp = e
   IF (temp.lt.0) THEN
     ...

Where does the non-portable round-off creep in?  That is, the non-portable
round-off in CASE that's different from the non-portable round-off in the
equivalent sequence of IF ... ELSE IF ...

-- 
vsnyder@jato.Jpl.Nasa.Gov
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vsnyder@jato.uucp