Re: LOOP

Am 07.07.2025 um 08:54 schrieb Gerry Jackson:

On 03/07/2025 20:33, minforth wrote:

On Sat, 28 Jun 2025 21:01:48 +0000, Anton Ertl wrote:
>

sean@conman.org writes:

 What is the difference between FOR/NEXT and DO/LOOP?  Don't they do the
same thing?

>
FOR ... NEXT on one system does not do the same thing as FOR ... NEXT
on some other systems, and they all behave different from DO ... LOOP.
>

>
Correct. Here are variants with iterators that even run on gforth 0.7.9:
>
\ ====== <n> FOR# .. #TIMES
==================================================
\ original: machine code
\ demo variant: slow Forth
>
: _ITERATE \ end xt
     swap
     BEGIN dup 0>
     WHILE over execute 1-
     REPEAT 2drop ;
>
: FOR# postpone [: ; IMMEDIATE
>
: #TIMES postpone ;] postpone _iterate ; IMMEDIATE
>
\ ====== <n> FOR .. N M .. NEXT
==============================================

 I've found looping quotations useful but I like to include the quotation inside the loop e.g. (without the syntactic sugar and moving the iterator inside the quotation):
 : downcount begin [: dup 0> if dup . 1- then ;] over 0> while execute repeat 2drop ;
10 downcount \ displays 10 9 8 7 6 5 4 3 2 1  ok
 Advantages are:
1) The xt is not passed to the quotation and so doesn't get in the way. 2) The xt is loaded as a literal when the quotation exits.
3) The quotation can exit the loop early by EXITing with 0 on the stack.

That's the nice thing about Forth: you can mould it to your taste. :-)
My initial motivation was that I wanted free access to the return
stack, and to kick UNLOOP out. From my slow playhorse Forth
(VM with address interpreter):
\ ------ Template: n FOR .. NEXT  and  adr n elsize FOR> .. NEXT
\ (negative elsize iterates backwards over array)
\ Primitives:
\ _ITERATE ( a n s xt -- ) M3 iterator (max. nesting depth 2)
\ N ( -- n ) N! ( n -- ) inner index in single FOR..NEXT loop
\ M ( -- n ) M! ( n -- ) inner index in nested FOR.FOR..NEXT.NEXT loops
: FOR>  postpone [: ; IMMEDIATE \ M3
: FOR 0 postpone literal postpone swap 1 postpone literal
postpone for> ; IMMEDIATE \ M3
: NEXT postpone ;] postpone _iterate ; IMMEDIATE \ M3
Demo session:
+---------------------+
¦  Min3rd Core Forth  ¦
+---------------------+
1020048 bytes free
# : T1 10 FOR n . NEXT ;  ok
# t1 0 1 2 3 4 5 6 7 8 9  ok
: t4 10 FOR -111 >r n . r> drop NEXT ;  ok
# t4 0 1 2 3 4 5 6 7 8 9  ok
# : T2 pad 5 cell FOR> n u. NEXT ;  ok
# : T3 pad 5 cell negate FOR> n u. NEXT ;  ok
# hex  ok
$ pad u. F7BD2F1C  ok
$ t2 F7BD2F1C F7BD2F20 F7BD2F24 F7BD2F28 F7BD2F2C  ok
$ t3 F7BD2F2C F7BD2F28 F7BD2F24 F7BD2F20 F7BD2F1C  ok
$ decimal  ok
# fload demo/for_next.m3
Benchmarking 1000000 loops:
DO..LOOP: 101.7 ms
FOR..NEXT: 26.5 ms ok
# : T5 3 FOR 7 >r 3 FOR 8 >r n . m . r> drop NEXT r> drop NEXT ;  ok
# t5 0 0 1 0 2 0 0 1 1 1 2 1 0 2 1 2 2 2  ok
#