Re: BETWEEN

On 12 Mar 2008 18:13:20 GMT, Albert van der Horst
<albert@[EMAIL PROTECTED]
> wrote:


> >> > : BETWEEN  ( n1|u1 n2|u2 n3|u3 -- flag )   OVER - -ROT - U< 0= ;

> In c within becomes
> 
> int within(int i, int j, int k)
> {
>     unsigned int ip=i;
>     unsigned int jp=j;
>     unsigned int kp=k;
> 
>     return
>         ( j<=i && i<k ) ||
>         ( jp<=ip && ip<kp ) ;
> }
> 
Doesn't have to. C unsigned arithmetic is guaranteed to wraparound,
so you can do the same x - l < u - l form if you like (and I do).

Except that C officially allows unsigned integers to not use the
signed form's sign bit e.g. 16-bit signed 0 to 32K-1 not 64K-1. There
are no known modern CPUs (and C compilers) on which this makes sense,
but in theory it is possible. In that case the unsigned-only form
doesn't work, and the mixed form needs to be more complicated. But
numbers in this form arguably don't match FORTH's semantics anyway.

> This is slightly clearer and also reveals that WITHIN is a very
> tricky two-for-the-price-of-one code from the 8-kbyte-for-4-users
> era.
> 
It's more explicit. If you understand modular arithmetic they're
equivalent (as already discussed).

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