Math.round() question for negative numbers

[Durgaprasad Guduguntla]

Hi,
In Chapter 6 of K&B book, it is mentioned that "If the number after the decimal point is less than 0.5, Math.round() is equal to Math.floor(). If the number after the decimal point is greater than or equal to 0.5, Math.round()is equal to Math.ceil()".
Please look at the following code:

  
public class MyClass 
{
public static void main(String argv[])
{
System.out.println(Math.round(-10.2)); //prints -10
System.out.println(Math.round(-10.5)); //prints -10
System.out.println(Math.round(-10.8)); //prints -11
System.out.println(Math.ceil(-10.8));  //prints -10.0
}
}

But I surprised to see the value of -10.8 is rounding to -11 which is a lower value than -10.8. The above rule seems to be okay with positive numbers, but does anyone explain what needs to be interpreted in case of negative numbers?
Thanks in advance.

[Cathy Song]

Hi,
The following approach is useful for Math.round()
if a is double
(long)Math.floor(a + 0.5d)
if a is float
(int)Math.floor(a + 0.5f)

Math.round (-10.8)
= (long) Math.floor (-10.8 + 0.5)
= (long) Math.floor (-10.3)
= (long) (-11.0)
= -11
Hope this helps!!
[ November 09, 2003: Message edited by: Cathy Song ]
[ November 09, 2003: Message edited by: Cathy Song ]

[Durgaprasad Guduguntla]

Thanks Cathy for quick reply. Your approach holds good for both negative and positive numbers. But rounding of a negative number to a lower value rather than to a next higher value looks strange behavior.

[Marlene Miller]

I don�t know the reason why they chose to make the half way point round the way it does, but this is the way I see it: I draw a number line.

            -3.6      -3.4
----+---------+----+----+---------+----
  -4.0           -3.5           -3.0
.
.
             3.4       3.6
----+---------+----+----+---------+----
   3.0            3.5            4.0

-3.5 and 3.5 both round to the RIGHT.
Otherwise the values between two whole numbers round to the nearest whole number.
[ November 09, 2003: Message edited by: Marlene Miller ]

[Harwinder Bhatia]

Originally posted by Durgaprasad Guduguntla:
But rounding of a negative number to a lower value rather than to a next higher value looks strange behavior.

Durga
Rounding gives the "closest" integer (this is by definition, strange or not). In case there's a "tie", the higher integer is chosen, eg.
rounding -3.5 gives -3.0 (as Marlene explained above), and mathematically, -3.0 > -3.5.
Cheers
Harwinder
[ November 09, 2003: Message edited by: Harwinder Bhatia ]

[Marlene Miller]

Ah, so that is how you handle these extraordinarilylongnames. You know how to truncate them. Durga.