Re: JESS: Rounding a float number

[Ernest Friedman-Hill]

Floating-point data types, like the Java ones used in Jess, are  
actually incapable of representing many decimal values exactly. Even a  
number as common as (for example) 0.11 can't be represented exactly,  
which is why in either Java or Jess, you'll get a result like this:

Jess> (/ 1.1 10)
0.11000000000000001

So what programmers typically do is carry around the full precision,  
then format the numbers for printing with the right number of decimal  
places.

If you really do need to carry precise values around -- for example,  
if your decimal values represent money -- then you can't use  
"float" (or RU.FLOAT). You have to use something more accurate. This  
is as true in Jess as it is in Java, C, C++, or any other language  
that uses IEEE floating point types or similar.

Two popular choices for "something more accurate" in Java are 1) for  
money, use integers to represent the number of whole cents, rather  
than decimal dollars, or 2) use the java.math.BigDecimal class, which  
gives exact values, but is computationally somewhat less convenient.



On Nov 29, 2009, at 10:01 AM, ANA TANASESCU wrote:

Hello everyone,

I have a fact that contains a float slot. The value for this slot is  
obtain by calculations.
Is there in Jess a function that rounds the float number to a  
specified number of decimal. I have searched into the Jess functions  
library and I found the round function, but it returns an integer  
number not a float number.
I  have, also, found the function format that uses the format  
specificators from C++ like this:
(bind ?a 9)
(bind ?b (/ ?a 13))
(printout t ?b )
(format t "%2.2f" ?b)

Instead of this I want that the value of the variable ?b to be  
converted to a number.

Best regards,
Ana




---------------------------------------------------------
Ernest Friedman-Hill
Informatics & Decision Sciences, Sandia National Laboratories
PO Box 969, MS 9012, Livermore, CA 94550
http://www.jessrules.com







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