[plt-scheme] Numerical precision

I ended following Richard's advice and round only when calculating  
interest. This a lot simpler that the parameterize solution and does  
the job.

(define (make-round:p numerical-precision)
   (define N (expt 10 numerical-precision))
   (define 1/N (/ 1 N))
   (lambda (x)
     (* 1/N (round (* x N)))))

(define round:p (make-round:p 2))

Thanks for all the input,

-- Jaime

On Mar 11, 2009, at 10:36 PM, Richard Cleis wrote:

> You want to do rounding or truncation according to the requirements  
> of the simulation.  He says that he is doing personal financial  
> simulations, which implies to me that after each operation the  
> simulation keeps only a certain precision (like a person would enter  
> only n digits beyond the decimal in a spreadsheet, or write them  
> from a calculator display.)
>
> Perhaps not :)
>
> rac
>
> On Mar 11, 2009, at 8:25 PM, Henk Boom wrote:
>
>> 2009/3/11 Richard Cleis <rcleis at mac.com>:
>>> You could consider computing everything with integers that  
>>> represent the
>>> precision in your sim.  (e.g., $1.23 would be 123 cents or 1230  
>>> mills or
>>> whatever.) After a division or multiply, rounding or truncation  
>>> can be used
>>> to create a new integer with the algorithm that your sim requires.
>>> Converting between the sim integers and $nn.nn needs to occur only  
>>> when you
>>> want to display values or accept input.
>>
>> This does not avoid the associativity problems though, nor the
>> problems of accumulation of error. You really want to do rounding  
>> only
>> when necessary.
>>
>>   Henk
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