Name: jk109818 Date: 02/19/2002


FULL PRODUCT VERSION :
J2SE 1.3.1 -b24

FULL OPERATING SYSTEM VERSION :
glibc-2.2.4-19.3
Linux (hostname) 2.4.9-6
Distribution RedHat 7.x

ADDITIONAL OPERATING SYSTEMS :
I have also tested this on an UtraSparc 10, and the same
thing happens.



A DESCRIPTION OF THE PROBLEM :
When performing modulo arithmetic on negative numbers the
current output is incorrect. For example -15%7 is
interpretted as being -1, it should be 6. Modulo should
never return a negative number. I probably would not have
ever run into this except for the fact that I have been
asked to write a Java implementation of the Enigma Machine
for a professor.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1.Simply use System.out.println(-15%7);


EXPECTED VERSUS ACTUAL BEHAVIOR :
I expected to obtain a positive number, by definition a
modulo should always be positive. Instead a -1 was reurned.

This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.lang.*;
import java.io.*;


class Test
{
    public static void main(String args[])throws IOException
    {

System.out.println("testing the output of -15 mod 7");

System.out.println(-15%7);

System.out.println((int)(-15 - Math.floor((double)-15/7)*7));
    }
}
---------- END SOURCE ----------

CUSTOMER WORKAROUND :
The work around for this, is to use the actual formula for
finding a modulo. As follows: a - floor(a/b) * b.
When doing this however you need to keep in mind that when
doing integer arithmetic that Java truncates during division.
So when you do Math.floor(a/b) with a < 0, you will get the
ceiling of that result. So the work around code should look
like this.
(int)(a - Math.floor((double)a/b)*b) this will produce the
floor of a/b when a<0 then cast the result back to an int,
for consistancy.
(Review ID: 139829)
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