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A common bug in published code

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Re: A common bug in published code

#3

Post by dalke »

The first few examples are fine. It's the unscaled ones later on, like "longarr[i] = (int) Math.random();" and especially "[Math.abs((int) Math.random()) % 3];" where the authors didn't notice that random() "returns a double value with a positive sign, greater than or equal to 0.0 and less than 1.0."

Here's another extreme example: (int) Math.random() / Integer.MAX_VALUE % (maxScoreCount + 1);

I eyeballed that about 5% of the calls are in error.

Re: A common bug in published code

#4

Post by nbpoole »

dalke wrote: post #3

The first few examples are fine. It's the unscaled ones later on, like "longarr[i] = (int) Math.random();" and especially "[Math.abs((int) Math.random()) % 3];" where the authors didn't notice that random() "returns a double value with a positive sign, greater than or equal to 0.0 and less than 1.0." Here's another extreme example: (int) Math.random() / Integer.MAX_VALUE % (maxScoreCount + 1); I eyeballed that about…

That's not true. Try running the following code:

    public class Test
    {
       public static void main(String[] args)
       {
               System.out.println("Test: " + (int)Math.random() * 100);
               System.out.println("Test: " + (int)(Math.random() * 100));
       }
    }
My results, from repeated tests:

    java Test
    Test: 0
    Test: 59

    java Test
    Test: 0
    Test: 18

    java Test
    Test: 0
    Test: 72

    java Test
    Test: 0
    Test: 11

Re: A common bug in published code

#5

Post by jimm »

dalke wrote: post #3

The first few examples are fine. It's the unscaled ones later on, like "longarr[i] = (int) Math.random();" and especially "[Math.abs((int) Math.random()) % 3];" where the authors didn't notice that random() "returns a double value with a positive sign, greater than or equal to 0.0 and less than 1.0." Here's another extreme example: (int) Math.random() / Integer.MAX_VALUE % (maxScoreCount + 1); I eyeballed that about…

They are all in error. "(int)Math.random()" always returns 0 because Math.random() returns [0.0, 1.0) (i.e., does not include 1). Casting that return value to a int will always return 0.

Re: A common bug in published code

#6

Post by [deleted] »

dalke wrote: post #3

The first few examples are fine. It's the unscaled ones later on, like "longarr[i] = (int) Math.random();" and especially "[Math.abs((int) Math.random()) % 3];" where the authors didn't notice that random() "returns a double value with a positive sign, greater than or equal to 0.0 and less than 1.0." Here's another extreme example: (int) Math.random() / Integer.MAX_VALUE % (maxScoreCount + 1); I eyeballed that about…

[deleted]

Re: A common bug in published code

#7

Post by [deleted] »

dalke wrote: post #3

The first few examples are fine. It's the unscaled ones later on, like "longarr[i] = (int) Math.random();" and especially "[Math.abs((int) Math.random()) % 3];" where the authors didn't notice that random() "returns a double value with a positive sign, greater than or equal to 0.0 and less than 1.0." Here's another extreme example: (int) Math.random() / Integer.MAX_VALUE % (maxScoreCount + 1); I eyeballed that about…

[deleted]

Re: A common bug in published code

#8

Post by jerfelix »

jimm wrote: post #5
dalke wrote: post #3

The first few examples are fine. It's the unscaled ones later on, like "longarr[i] = (int) Math.random();" and especially "[Math.abs((int) Math.random()) % 3];" where the authors didn't notice that random() "returns a double value with a positive sign, greater than or equal to 0.0 and less than 1.0." Here's another extreme example: (int) Math.random() / Integer.MAX_VALUE % (maxScoreCount + 1); I eyeballed that about…

They are all in error. "(int)Math.random()" always returns 0 because Math.random() returns [0.0, 1.0) (i.e., does not include 1). Casting that return value to a int will always return 0.

They are not in error if you are trying to get a 0. This is yet another good technique for obfuscated code.

Re: A common bug in published code

#9

Post by nbpoole »

jerfelix wrote: post #8
jimm wrote: post #5

Earlier quoted context omitted.

They are all in error. "(int)Math.random()" always returns 0 because Math.random() returns [0.0, 1.0) (i.e., does not include 1). Casting that return value to a int will always return 0.

They are not in error if you are trying to get a 0. This is yet another good technique for obfuscated code.

Indeed. Take a look at http://www.google.com/codesearch/p?hl=en#TXS-ndgbCls/trunk/j... for instance: if you weren't looking for an incorrect cast, it would be very hard to notice. Two out of three casts are correct: only one is missing the parentheses.

Re: A common bug in published code

#10

Post by fjh »

dalke wrote: post #3

The first few examples are fine. It's the unscaled ones later on, like "longarr[i] = (int) Math.random();" and especially "[Math.abs((int) Math.random()) % 3];" where the authors didn't notice that random() "returns a double value with a positive sign, greater than or equal to 0.0 and less than 1.0." Here's another extreme example: (int) Math.random() / Integer.MAX_VALUE % (maxScoreCount + 1); I eyeballed that about…

Have a look at the precedence table for Java: http://introcs.cs.princeton.edu/11precedence/

The cast has higher precedence than multiplication, so the scaling doesn't help at all, unless you use parenthesis to force the right order of evaluation.

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