It was thus said that the Great dxf <[email protected]> once stated:

<[Wil] W.BADEN1> Chuck walked out of ANSI becuz it wudn't make
FOR...NEXT required.
<[Wil] W.BADEN1> That was 1988. And the operative word is "required."

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

-spc

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[email protected] 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.

- anton
--
M. Anton Ertl http://www.complang.tuwien.ac.at/anton/home.html
comp.lang.forth FAQs: http://www.complang.tuwien.ac.at/forth/faq/toc.html
New standard: https://forth-standard.org/
EuroForth 2023 proceedings: http://www.euroforth.org/ef23/papers/
EuroForth 2024 proceedings: http://www.euroforth.org/ef24/papers/

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

[email protected] 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
==============================================
\ original: machine code with circular iteration control stack
\ advantage: avoids UNLOOP and doesn't clutter rstack
\ demo variant: slow Forth, indices on rstack like DO..LOOP, no
advantage

\ for gforth:
: rpick rp@ swap 1+ cells + @ ;

: N 2 rpick ; ( -- inner-index )
: M 6 rpick ; ( -- outer-index )

: _N-ITERATE \ end xt --
swap >r 0 >r
BEGIN 2r@ u> \ end n f
WHILE dup execute r> 1+ >r
REPEAT 2r> 2drop drop ;

: FOR postpone [: ; IMMEDIATE

: NEXT postpone ;] postpone _n-iterate ; IMMEDIATE

\ ====== <array> <#el> <elsize> FOR{ .. N M .. }NEXT
=========================
\ original: machine code with circular iteration control stack
\ advantage: avoids UNLOOP and doesn't clutter rstack
\ demo variant: slow Forth, indices on rstack like DO..LOOP, no
advantage

: _ARR-ITERATE \ adr els step xt --

r dup >r * over + swap 2r> 2swap 2>r \ xt size -- s | r: end arr

BEGIN 2r@ u>
WHILE over execute dup r> + >r
REPEAT 2r> 2drop 2drop ;

: FOR{ postpone [: ; IMMEDIATE

: }NEXT postpone ;] postpone _arr-iterate ; IMMEDIATE

\ --- Tests

: STARS
20 FOR# '*' emit #TIMES ;
: NUMBER1
10 FOR n . NEXT ;
: NUMBER2
3 FOR cr 4 FOR n m 1+ * . NEXT NEXT ;
: TYPE-CHARARRAY \ a u --
dup >r pad swap move
pad r> 1 FOR{ n c@ emit }NEXT ;

CR STARS
CR NUMBER1
CR NUMBER2
CR S" Fortune" TYPE-CHARARRAY

--

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

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

[email protected] 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.

--
Gerry

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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:

[email protected] 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
#

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