retroforth/doc/book/techniques/loops

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# Loops
RETRO provides several words for creating loops.
## Unconditional Loops
An unconditional loop begins with `repeat` and ends with `again`.
:test repeat #1 n:put sp again ;
test
Unconditional loops must be inside a definition or quote. To exit
one of these, use `0;`, `-if;` or `if;`.
:test #100 repeat 0; dup n:put sp n:dec again ;
test
:test #100 repeat dup #50 eq? [ 'done! s:put nl ] if; n:dec again ;
test
You can also achieve this via recursion:
:test 0; dup n:put sp n:dec test ;
#100 test
Be careful with recursion as the virtual machine will have a limited
amount of space for the address stack and recursing too many times
can cause a stack overflow.
## Conditional Loops
There are two conditional looping combinators: `while` and `until`.
Both take a quote and execute it, checking a returned flag to decide
when to stop running.
#0 [ dup n:put sp n:inc dup #10 eq? ] until
#10 [ dup n:put sp n:dec dup n:-zero? ] while
## Counted Loops
There are two combinators for counted loops. These are `times` and
`times<with-index>`.
#0 #10 [ dup n:put sp n:inc ] times nl
#10 [ I n:put sp ] times<with-index>
The `times<with-index>` provides an index via the `I`, `J`, and
`K` words. `I` will be the index of the current loop, with `J` and
`K` being the indexes of the next two older loops.
The loop indexes can be accessed outside the loop body:
:display I n:square n:put sp ;
:squares [ display ] times<with-index> nl ;
#100 squares
## Tradeoffs
The unconditional loop form is more efficient as it's just a
simple jump operation. The `times` counted loops are a little
slower, but can be cleaner and more readable in many cases. The
`times<with-index>` form is significantly slower than the other
two forms.