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Comment by baltcode | Hacker News Reader

Apparently, python only has 5 instances of the corresponding error: http://www.google.com/codesearch?hl=en&lr=&q=\s%2Bin...

Python-Java flame-war, anyone?

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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.)

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).

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

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

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

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

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...

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

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.

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

> 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

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>

In that case you can always use floor or ceil.

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.

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.

There is no need to cast to int in Python:

    a = random.randrange(10000)

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