Re: Decimal rounding of a float data type in Java.

["James Donkin" <James@xxxxxxxxxxxxxxx>]

> Well, not exactly.  Your "average" doesn't become 4.22, it becomes
something
> that's extremely close to 4.22.  Floating point numbers can't express
*any*
> fraction exactly unless it's a power of 0.5.  So don't be surprised if you
> do "System.out.println(average)" and you find something like
> "4.2200000000033775" appearing.
e.g. average = 422 / 100;

This will always give 4.22. The time you get problems is with 323 / 34 or
some other fraction which generates an irrational number. Saying that

    float average = 422 / 100;

 will give 4.22000032 is like saying that

    float average = 4.22;

 will leave average = 4.2200042. Dividing by 100 just generates a two digit
shift.

----- Original Message -----
From: Clapham, Paul <pclapham@core-mark.com>
To: <JAVA400-L@midrange.com>
Sent: Thursday, March 01, 2001 4:56 PM
Subject: RE: Decimal rounding of a float data type in Java.


> Well, not exactly.  Your "average" doesn't become 4.22, it becomes
something
> that's extremely close to 4.22.  Floating point numbers can't express
*any*
> fraction exactly unless it's a power of 0.5.  So don't be surprised if you
> do "System.out.println(average)" and you find something like
> "4.2200000000033775" appearing.  That was the problem experienced in the
> original post, I believe.  You can mask the problem by using a
DecimalFormat
> object to display the number, or you can use BigDecimal objects, but in
> either case you may get burned by rounding that you didn't expect.
> (BigDecimal provides a half-dozen different ways to round.)
>
> In commercial work this isn't likely to be much of a problem, but in
> scientific work it can be, and there's a whole field of mathematics
devoted
> to this kind of thing.
>
> PC2
>
> -----Original Message-----
> From: James Donkin [mailto:James@DuMaurier.co.uk]
> Sent: March 1, 2001 05:49
> To: JAVA400-L@midrange.com
> Subject: Re: Decimal rounding of a float data type in Java.
>
> </snip>
>
> One way for getting rid of the rounding problem would be to...
>
>     float roundedInt = Float.parseFloat(twoDigits.format( average ));
>
> Or you could...
>
>     int temp = (int)(average * 100);
>     average = (float)temp / 100;
>
> e.g. average = 4.222222222222
>       temp then =  422
>       average then becomes 4.22
>
> </snip>
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