Apparently, python only has 5 instances of the corresponding error: http://www.g...

myelin · on April 21, 2011

Slight difference here. There are actually two types of errors in the Java code:

1. int foo = (int) Math.random() * some_max_value;

The error here is assuming that the multiplication takes place before the truncation. This isn't happening in the Python code because int(expression to truncate) is unambiguous. (+1 to Python here for making it hard to shoot yourself in the foot).

2. int foo = (int) Math.random();

The error here is assuming that Math.random returns something outside [0.0, 1.0). This is the error that all five of the Python examples are showing. (Boo to Python AND Java programmers.)

ZoFreX · on April 22, 2011

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 · on April 22, 2011

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

GVRV · on April 22, 2011

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 · on April 22, 2011

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

ZoFreX · on April 22, 2011

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 · on April 22, 2011

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

nostrademons · on April 22, 2011

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

Dilpil · on April 22, 2011

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

llimllib · on April 22, 2011

> 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 · on April 22, 2011

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 · on April 22, 2011

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

dangoldin · on April 22, 2011

In that case you can always use floor or ceil.

mirkules · on April 22, 2011

Clearly, PHP is a better language than both: int rand($min, $max)

ducks

All joking aside, I'm just pointing out the absurdity of comparing one language to another based on how easy it is to shoot oneself in the foot.

njharman · on April 22, 2011

I can help that flame war...

I bet those 5 were written by folks with more (or stronger) skill with C/C++/Java other strongly typed language where casting is required/common.

Casting is uncommon and "weird" in Python. Usually means you're being unpythonic. As in this case you should be using randint or randrange rather than cast to int.

rbonvall · on April 21, 2011

There is no need to cast to int in Python:

    a = random.randrange(10000)