Re: LOOP

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.
--
Gerry