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