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[forth/en] Clarify bounds of do and the difference between ?do vs do (#4058)

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* Clarify bounds of do and ?do vs do

* Re-introduce square into example

---------

Co-authored-by: ven <vendethiel@hotmail.fr>
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Matthew D. Scholefield 2023-12-14 06:56:16 -08:00 committed by GitHub
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@ -93,8 +93,8 @@ see square \ : square dup * ; ok
\ ------------------------------------ Loops -----------------------------------
\ `do` is also a compile-only word.
: myloop ( -- ) 5 0 do cr ." Hello!" loop ; \ ok
\ `?do` is also a compile-only word.
: myloop ( -- ) 5 0 ?do cr ." Hello!" loop ; \ ok
myloop
\ Hello!
\ Hello!
@ -102,16 +102,17 @@ myloop
\ Hello!
\ Hello! ok
\ `do` expects two numbers on the stack: the end number and the start number.
\ `?do` expects two numbers on the stack: the end number (exclusive) and the
\ start number (inclusive).
\ We can get the value of the index as we loop with `i`:
: one-to-12 ( -- ) 12 0 do i . loop ; \ ok
one-to-12 \ 0 1 2 3 4 5 6 7 8 9 10 11 ok
\ `?do` works similarly, except it will skip the loop if the end and start
\ numbers are equal.
: squares ( n -- ) 0 ?do i square . loop ; \ ok
10 squares \ 0 1 4 9 16 25 36 49 64 81 ok
\ `do` works similarly, except if start and end are exactly the same it will
\ loop forever (until arithmetic underflow).
: loop-forever 1 1 do i square . loop ; \ ok
loop-forever \ 1 4 9 16 25 36 49 64 81 100 ...
\ Change the "step" with `+loop`:
: threes ( n n -- ) ?do i . 3 +loop ; \ ok