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From: melahi_ahmed@yahoo.fr (ahmed)
Newsgroups: comp.lang.forth
Subject: Re: Back & Forth - Co-routines
Date: Sat, 1 Feb 2025 08:22:01 +0000
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On Sat, 1 Feb 2025 7:26:11 +0000, Anton Ertl wrote:

> Paul Rubin <no.email@nospam.invalid> writes:
>>This seems more in the locals spirit:
>>
>>: blend { a x b -- n } 100 b x - b a - */ ;
>>: tri_mf3.1 { x a b c -- mf }
>>    a x <= x b < AND IF b x a blend EXIT THEN
>>    b x <= x c < AND IF b x c blend EXIT THEN
>>    0 ;
>
> Forth has a word WITHIN that should come in handy here:
>
> : tri_mf3.1 { x a b c -- mf }
>     x a b within IF b x a blend EXIT THEN
>     x b c within IF b x c blend EXIT THEN
>     0 ;

Yes, it works fine. Thanks.

> Another alternative, assuming a<=b<=c:
>
> : tri_mf3.1 { x a b c -- mf }
>     case
>       x a < ?of 0 endof
>       x b < ?of b x a blend endof
>       x c < ?of b x c blend endof
>       0 0
>     endcase ;

It also works fine without problems.


>
> The disadvantage in the latter case is that the above and below cases
> are separate, needing another branch and possibly increasing the
> number of mispredictions.  This can be addressed with:
>
> [undefined] select [if]
> : select ( u1 u2 f -- u )
>   if swap then nip ;
> [then]
>
> : tri_mf3.1 { x a b c -- mf }
>     x a c within if
>       b x  x b < a c select  blend
>     else
>       0
>     then ;

This latter gives wrong results:

-100 -50 0 50 tri_mf3.1 . 0  ok    \ this is true
-50 -50 0 50 tri_mf3.1 . -4900  ok \ false, must be 0
-10 -50 0 50 tri_mf3.1 . -900  ok  \ false, must be 80
0 -50 0 50 tri_mf3.1 .             \ here division by zero
*the terminal*:47:12: error: Division by zero
0 -50 0 50 >>>tri_mf3.1<<< .

I think it must be:
(the true/false flag must on tos for select as you defined it)

: tri_mf3.1 { x a b c -- mf }
    x a c within if
      b x a c x b < select  blend
    else
      0
    then ;

and this one works fine.

-100 -50 0 50 tri_mf3.1 . 0  ok
-50 -50 0 50 tri_mf3.1 . 0  ok
-10 -50 0 50 tri_mf3.1 . 80  ok
0 -50 0 50 tri_mf3.1 . 100  ok
10 -50 0 50 tri_mf3.1 . 80  ok
50 -50 0 50 tri_mf3.1 . 0  ok
100 -50 0 50 tri_mf3.1 . 0  ok

>
> In Gforth (development) SELECT is a primitive which hopefully does not
> branch; in that case you have only one branch here.
>
> - anton


Ahmed

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