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

["Clapham, Paul" <pclapham@xxxxxxxxxxxxx>]

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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