On: A common bug in published code

[ZoFreX]

Third error: This is not the correct way to randomly pick a number in a set range. The proper way is actually quite complicated. Imagine you do (int) (Math.random() * 10), this could give you numbers from 0 to 10. However, you only get 0 if Math.random() * 10 is less than 0.5, but you get 1 if the value is between 0.5 and 1.5. You are half as likely to see a zero!

I can't speak for Python, but in Java it's quite simple to do it right; Random#nextInt(int) "Returns a pseudorandom, uniformly distributed int value between 0 (inclusive) and the specified value (exclusive)" (also consider using SecureRandom).

[thedufer → ZoFreX]

We're all aware that casting to an integer does truncation, not rounding. Right?

[GVRV → ZoFreX]

At least in Java and other languages, when casting as int truncates, and doesn't round the number, so: ((int) (0.9999999)) == 0 But then there's Math.floor and Math.ceil for what you're describing which can be used as well.

[anonymoushn → ZoFreX]

  public class example
  {
      public static void main(String[] JAVA_LOL) throws Throwable
      {
          // prints "6"
          System.out.println((int)6.99999);
      }
  }

[ZoFreX → anonymoushn]

You (and everyone else) are of course right. I remembered the wrong thing, what I said doesn't apply to that method in Java. What does apply is that there are lots of subtleties in generating random numbers, and there's rarely a reason to re-invent the wheel. There's a good example of how subtle pseudorandomness is in the Java documentation, so I will link that instead of embarrassing myself further: http://download.oracle.com/javase/1.4.2/docs/api/java/util/R...

[calpaterson → ZoFreX]

I actually struggle to understand why people even use Math.random() when the Random class exists.

[nostrademons → calpaterson]

Don't need an import for Math.random(). Random is in java.util, while Math is in java.lang.

[Dilpil → calpaterson]

Alot of people probably look for Random but find Math.random().

[llimllib → ZoFreX]

> I can't speak for Python, but in Java it's quite simple to do it right

http://docs.python.org/library/random.html#random.randint

[streety → llimllib]

Actually the python equivalent of "Returns a pseudorandom, uniformly distributed int value between 0 (inclusive) and the specified value (exclusive)" would be random.randrange(0, n, 1)

random.randint(a, b) returns a uniformly distributed int value between a (inclusive) and b (inclusive).

</nitpick>

[MBlume → ZoFreX]

import random # will print an integer between 0 and 10 inclusive print randint(0,10)

[dangoldin → ZoFreX]

In that case you can always use floor or ceil.