Rocco Moretti <[EMAIL PROTECTED]> writes:
>> My "favorite" infinte loop with while is:
>> i = 0
>> while i < 20:
>> do_process(i)
>> Note the prominent *lack* of any change to i here?
>> Oh, for:
>> from i = 0
>> invariant 0 <= i <= 20
>> variant 21 - i
>> until i > 19
>> loop
>> do_process(i)
>> which throws an exception at the beginning of the second loop.
>
> What language is that from?
Eiffel.
> I take it the exception is from the "21-i" not changing as it goes
> around the loop, right? (But why can't "variant i" work just as well?)
Because that's the way variant is defined. You actually want the
variant to move in a specific direction, so that you don't wind up
incrementing it when you should be decrementing it. Eiffel chose down,
so you you need -i at the very least. It also insists that the
invariant be positive. I'm not sure why; it makes the variant more
complicated, resulting in exceptions from bugs in the variant as well
as bugs in the loop.
<mike
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Mike Meyer <[EMAIL PROTECTED]> http://www.mired.org/home/mwm/
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