Re: [Re: RPGILE V4.3 Gotcha]

[Jim Langston <jlangston@xxxxxxxxxxxxxxxx>]

Like I was saying, 65100 stored in a SIGNED integer is -436.  Unsigned it would
be 65100.

Regards,

Jim Langston

Joel Fritz wrote:

> My recollection from turbo c++ back in 91 or so, that was the way integer
> overflow worked.  I think unsigned integers just wrapped which is the
> behavior you're seeing except it's going through the two's complement
> translation into negative numbers.  It's actually a lot like RPG truncation
> because the leftmost binary digit goes to the bit bucket.
> e.g. 1111 1111 1111 1111 + 0000 0000 0000 0001 = 0000 0000 0000 0000
>
> My C teacher's favorite saying was "C doesn't care."
>
> > -----Original Message-----
> > From: Jim Langston [mailto:jlangston@conexfreight.com]
> > Sent: Friday, October 08, 1999 12:32 PM
> > To: RPG400-L@midrange.com
> > Subject: Re: [Re: RPGILE V4.3 Gotcha]
> >
> >
> > You forgot something, the range of a short integer in C is
> > -32768 to 32767
> > because of the sign bit.  When you assign the value 65100 to
> > the variable
> > A it is taking the high bit (32768) and making it the sign bit.
> >
> > Also, in Intel integers are stored in what is called "2's
> > compliment".  The bit's
> >
> > are reversed and 1 is added (don't ask me why, it was
> > explained to me once
> > but didn't make much sense).  The effect of this is: take
> > your number and
> > calculate what it would be minus the high bit, 32768, and it
> > comes out to
> > 32332.  32768 - 32332 is 436.  Plus the sign bit makes it
> > -436, which is your
> > answer.
> >
> > In C you can assign between characters, integers and hexadecimal with
> > out any complaint from the compiler.  The compiler doesn't
> > care if you give
> > it the hexadecimal format (0x1234 or whatever) or the decimal
> > format (65100)
> > it happily goes along and does exactly what you told it, it
> > assigns the bit
> > pattern
> > you gave it to the variable.
> >
> > To check this yourself, assign a value lower than 32767 to
> > the variable A, such
> > as 32000, and by multiplying it by 100 it should overflow the
> > integer (making it
> > 3200000) then divide by 100, and it would come back to 32000.
> >
> > I am actually surpassed by this, however, as I would of
> > thought that you would of
> >
> > gotten an over flow error, but I guess C is converting it to
> > a long in the math
> > when
> > it sees it has to, then type casting it back to a short to assign it.
> >
> > Regards,
> >
> > Jim Langston
> >
> > Buck Calabro wrote:
> >
> > > > >After 5 or 6 languages, I still usually check to make sure
> > > >they are doing it the same way.  And so far I haven't run
> > > >across any problems, not even with the MULT statement
> > > >in RPG.
> > >
> > > Actually, RPG was always different from other languages
> > because of the fixed
> > > size of the numbers.  We never much thought about it
> > because MULT and DIV
> > > cannot produce intermediate results.  I believe that EVAL
> > works very much
> > > the same as other languages.  Take this C code that I just
> > tried on the 400:
> > >
> > > #include <stdio.h>
> > > void main () {
> > >
> > > short a;
> > > short b;
> > > long  c;
> > >
> > > a = 65100;
> > > b = 100;
> > > c = a * b / 100;
> > > printf("result=%i\n", c);
> > >
> > > }
> > >
> > > Although "natural" arithmetic tells you that the result
> > should be 65100, the
> > > result is actually -436.  I have no run-time error, nor do I have a
> > > compile-time error warning me that I might lose precision.
> > I am certainly
> > > not versed in too many languages, but it seems to me that
> > any language
> > > allowing expressions will exhibit similar behaviour.
> > >
> > > Buck Calabro
> >
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