[plt-scheme] inexact->exact

From: Katsmall T. Wise, Esquire (kela_bit at netvision.net.il)
Date: Sun May 25 05:31:39 EDT 2003

Why do you need inexact arithmetics?
Moth arithmetics are quite possible with exact numbers.
Could you describe your problem?
Perhaps I could help make it exact.

Katsmall T. Wise, Esquire

John Clements wrote:

>  For list-related administrative tasks:
>  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> On Friday, May 23, 2003, at 18:57 US/Eastern, Alex Peake wrote:
>>   For list-related administrative tasks:
>>   http://list.cs.brown.edu/mailman/listinfo/plt-scheme
>> Is there a faster way to convert an inexact integer (4.0) to an exact 
>> integer (4)?
>> (Context: I am using the result of floating point math to index into 
>> a vector for a translation
>> value)
>> I tried inexact->exact and it is very slow. I can find no other 
>> references in Help Desk (with my
>> search guesses anyway).
> I could very well be wrong, but I'd guess there's no built-in way to 
> speed this up.  In the absence of static reasoning about the flow of 
> values, there's no way that mzscheme can guarantee safety without 
> performing run-time checks.
> If this is a real performance issue for you (and it sounds like it 
> is), your best bet is probably to write a tiny C extension that 
> performs the cast for you unsafely.  To do the job right, what you'd 
> really like is to write a small tool that can prove statically that 
> the values that flow into this extension are always of a certain type 
> (presumably a double or float). In fact, while you're at it, you might 
> as well chop out another safety check by doing the vector lookup (you 
> still need a bounds check, of course) while in the C code.
> hope this helps,
> john clements

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