Can scala.util.Random.nextInt (): Int occasionally return a negative value?

Question

Documentation for scala.util.Random.nextInt (n: Int): Int says "Returns a pseudorandom, uniformly distributed int value between 0 (inclusive) and the specified value (exclusive)..." while for scala.util.Random.nextInt (): Int it says "Returns the next pseudorandom, uniformly distributed int value...", without saying anything about the zero. Can I get a negative value here occasionally?

Where does it say that? The docs I found said "All 2^32 possible int values are produced with (approximately) equal probability", which means that negative numbers are *more* likely than positive ones (since there is one more negative number). — Michael Lorton, Oct 16 '11 at 17:18

om-nom-nom · Answer 1 · 2011-10-16T11:30:04.357

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Yes, you can (and that's ok due to definition of uniform distribution). Moreover you'll get it in nearly 50% of cases.

(for(i <- 1 to 100000) yield scala.util.Random.nextInt()).filter(_<0).length

have yielded for me 49946 - that's quite close to the 50%.

edited Oct 16 '11 at 11:30

answered Oct 16 '11 at 11:24

om-nom-nom

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On average, in fact, *exactly* 50% of cases – Luigi Plinge Oct 16 '11 at 12:28
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@Luigi Don't forget about neutral cases, when random value is equals to 0. So there is would be 49,9...% percents of positive, the same number of negatives and a few of zeros. – om-nom-nom Oct 16 '11 at 12:32
I didn't forget... well, I'm assuming any value is equally likely. Maybe you can't get Int.MinValue or something. There are 2147483648 negative, 2147483647 positive, and 1 zero Int – Luigi Plinge Oct 16 '11 at 12:34

score 10 · Accepted Answer · answered Oct 16 '11 at 11:26

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Apparently, yes. It returned a negative value on my first try! :-)

scala> import util.Random
import util.Random

scala> Random.nextInt
res0: Int = -299006430

answered Oct 16 '11 at 11:26

missingfaktor

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what are the odds of that?! :) – Fabien Warniez Oct 02 '17 at 18:38
@FabienWarniez, what are the odds of you commenting this on my ancient post yesterday, and me running into similar situation at about the same time? :) https://twitter.com/missingfaktor/status/914642815854006272 – missingfaktor Oct 03 '17 at 18:55
I am a little bit spooked TBH! – missingfaktor Oct 03 '17 at 18:55
This answer from 2011 was the second search result when I looked for `scala random int non-zero`. Maybe it's a sign. – Joseph Yaduvanshi Oct 04 '17 at 03:25

score 10 · Answer 3 · edited Jul 25 '14 at 15:53

As you can see here (using Mike Harrah's excellent sxr), Scala's Random just delegates to an underlying java.util.Random, which is referred to as self.

As others pointed out the default range is between Integer.MIN_VAL and Integer.MAX_VAL, in other words, any Integer possible, including the negative ones.

If you want just the positive range, you can use the overloaded method nextInt that takes an argument, like this:

Random.nextInt(Integer.MAX_VALUE);

According to the docs:

Returns a pseudorandom, uniformly distributed int value between 0 (inclusive) and the specified value (exclusive), drawn from this random number generator's sequence.

If you want the positive range, you'd rather do: `Random.nextInt(Integer.MAX_VALUE+1)`, since 0 is not considered positive and Integer.MAX_VALUE is going to be excluded at the top end. — swifthorseman, Jun 20 '18 at 10:30

Clark Bao · Answer 4 · 2011-10-16T13:19:07.137

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scala.uti.Random.nextInt (): Int leverages the same method of java.util.Random. And the range as Luigi pointed out is [Integer.MIN_VAL,Integer.MAX_VAL]. Actually "uniformly distributed int value" means any number of int type is possible to be returned and the chance of each one in theory to be the same.

edited Oct 16 '11 at 13:19

answered Oct 16 '11 at 13:00

Clark Bao

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Can scala.util.Random.nextInt (): Int occasionally return a negative value?

4 Answers4