[Jscheme-devel] FAQ entry: (* 256 256 256 256) --> 0

[Timothy J. H. <ti...@cs...>]

On Aug 5, 2004, at 12:57 PM, Geoffrey Knauth wrote:

> Maybe this is a FAQ.  I guess I was surprised I got 0 instead of #f 
> and/or an exception of some kind.
>
> $ java -cp lib/jscheme.jar jscheme.REPL
> JScheme 7.1 (8/5/04 3:06 AM) http://jscheme.sourceforge.net
> > (* 256 256 256 256)
> 0
> > (* 256 256 256)
> 16777216
> > (* 256 256 256 128)
> -2147483648
> >

I guess we need a FAQ for JScheme and questions about numerics need to 
be there...

---------------------

Q: Why does integer arithemetic give numerically incorrect results in 
JScheme?

A: JScheme numbers and operators are taken from Java so, for example,
     256 in an "int" and hence raising 256 to the fourth power gives 
zero in JScheme

  >      (display (* 256 256 256 256))
   0

  This gives zero because it gives the same result as a corresponding 
Java program,
  such as the following:

  public class Demo {
     public static void main(String[] args) {
        int x = 256;
         System.out.println(x * x * x * x);
     }
    }

Java integer arithmetic is does not throw exceptions on overflow and
just returns the lower order 32 bits of the answer, hence you get 0 for

If you want the Scheme numeric tower (which is an optional part of 
R4RS) you
can import the SISC tower and use the siscnum.scm module described here:
      http://localhost:8080/jscheme/src/using/package.html
However, if you use this you must explicity convert SISC numbers to 
Java numbers
before using those numbers in javadot calls.

---Tim---



>
> Geoffrey
> --
> Geoffrey S. Knauth | http://knauth.org/gsk
>
>
>
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