[MATH-724] RandomDataImpl.nextInt does not distribute uniformly for negative lower bound

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 1.0, 1.1, 1.2, 2.0, 2.1, 2.2
• Fix Version/s: 3.0
• Assignee: Unassigned
• Reporter: Dennis Hendriks
• Created: 12/Dec/11 15:03
• Updated: 24/Mar/12 16:16
• Resolved: 20/Dec/11 21:14

Description

When using the RandomDataImpl.nextInt function to get a uniform sample in a [lower, upper] interval, when the lower value is less than zero, the output is not uniformly distributed, as the lowest value is practically never returned.

See the attached NextIntUniformTest.java file. It uses a [-3, 5] interval. For several values between 0 and 1, testNextIntUniform1 prints the return value of RandomDataImpl.nextInt (as double and as int). We see that -2 through 5 are returned several times. The -3 value however, is only returned for 0.0, and is thus under-respresented in the integer samples. The output of test method testNextIntUniform2 also clearly shows that value -3 is never sampled.