Re: [Re: RPGILE V4.3 Gotcha]

[Jim Langston <jlangston@xxxxxxxxxxxxxxxx>]

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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