A common bug in published code

http://www.google.com/codesearch?lr=&q=$int$\s*Math\.random

by shawndumas • 15 years ago

140 71 15 years ago

dalke2• 15 years ago

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.

nbpoole2• 15 years ago

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

electrum• 15 years ago

It is random! http://xkcd.com/221/

jimm• 15 years ago

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.

jerfelix• 15 years ago

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

ubernostrum2• 15 years ago

A lot of the results I'm seeing in the search have more problems with order of operations -- so long as you multiply the random() result by a good-sized constant before you cast to int, it actually does what it's "supposed" to do.

fjh• 15 years ago

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.

dalke2• 15 years ago

Oh, wow! I didn't know that about Java. I see then why I got all the downvotes.

ilcavero• 15 years ago

the main cause here is that randomness is hard to test so that code was just assumed to be correct. With randomness you can never be sure, http://www.random.org/analysis/

klodolph• 15 years ago

Yes, but... (int)Math.random() always returns zero. A little suspicious.

polynomial• 15 years ago

You can never be sure the next one won't be a 1!

ignifero2• 15 years ago

A-mazing

Also, notice the difference in number of results between C and C++ below:

http://www.google.com/codesearch?hl=en&lr=&q=if%5Cs*...

benburleson• 15 years ago

That's why I write comparisons like

if (CONSTANT == variable)

klochner• 15 years ago

"yoda conditions":

http://stackoverflow.com/questions/2349378/new-programming-j...

derleth2• 15 years ago

Yoda conditions, those are called.

http://united-coders.com/christian-harms/what-are-yoda-condi...

Splines• 15 years ago

I know it's safer, but personally I find it harder to read. Especially if the constant is a #define or enum and is similar in format to the variable in question.

loumf• 15 years ago

The code is generating code for a different language -- the = is always in a string.

learner4life• 15 years ago

Maybe I am feeling dense, but I do not understand what you are trying to point out. None except one are errors as they relate to embedded sql statements (where Equal To is the comparison operator).

ignifero2• 15 years ago

maybe we re seeing different results. mine have 8 incorrect assignments in the first page

samps• 15 years ago

Someone should write a tool that scrapes these results and automatically files bugs at the associated bug trackers for open-source projects.

bnewbold• 15 years ago

I've been thinking of doing something and i'm looking for people to work with! Notes here: http://memeschemes.com/code_search/

mbenjaminsmith• 15 years ago

A bug in that program would certainly be wonderful. Have fun when it files a few million unnecessary bug reports while you're sleeping.

bobds• 15 years ago

I've had some similar ideas myself, consider me interested. I've written some (very little) code to search projects among the big open source hosts (SF, Google, Github etc). Also have a nice domain for it.

wladimir• 15 years ago

This will be very useful as long as it finds confirmed bugs. Otherwise it will be more like an unasked-for code style check (For example, one can argue using functions like strcpy are unsafe, but unless it's really possible to get too many characters in the buffer, it's not a bug)

endian• 15 years ago

I've been wanting to do that for:

* NPM packages whose package.json file is missing a link to the package's GitHub repo

* Markdown READMEs without the right file extension (for GitHub)

ghewgill• 15 years ago

Last year I found several instances of the printf format "200%d" when dealing with years. Some still exist: http://hewgill.com/journal/entries/548-y2010-bugs-found-via-...

dgallagher• 15 years ago

I think that's called job security in some circles. ;)

baltcode• 15 years ago

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?

myelin• 15 years ago

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.)

ZoFreX2• 15 years ago

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).

rbonvall• 15 years ago

There is no need to cast to int in Python:

    a = random.randrange(10000)

njharman• 15 years ago

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.

mirkules• 15 years ago

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.

Adrock• 15 years ago

See the previous HN discussion: http://news.ycombinator.com/item?id=960886

__mark• 15 years ago

Oh, so it always rounds down to zero, that's pretty bad. Casting it to a double would help right?

Thasc• 15 years ago

Instead of

int runs = (int) Math.random() * 1000000;

You'd do

int runs = (int) (Math.random() * 1000000);

Confusion• 15 years ago

That would be pointless, because Math.round already returns a double.

nicetryguy• 15 years ago

im floored

modulow• 15 years ago

i see what you did there...

mattdeboard• 15 years ago

Yeah, I nearly hit the ceiling myself.

richardw• 15 years ago

Random ints between 0 and 1 offer a pretty limited range of jokes, I guess.

http://en.m.wikipedia.org/wiki/Range_(computer_science) Edit: added range def because of downvotes. Sigh

mturmon• 15 years ago

Your pun is roundly appreciated.

ChuckMcM• 15 years ago

One would hope at some point a tool would warn you about this since clearly it could optimize (int) Math.random() to just 0.

[queue debate about tools that hold your hands vs understanding what you are actually writing]

I wonder if they did the search for if (x = y) bug pre-gcc-4.x-warn what sort of numbers they would get.

derleth2• 15 years ago

> queue

Speaking of common bugs...

(The word you're looking for is 'cue'.)

nolanw• 15 years ago

Were you actually confused, or are you just picking nits? If the latter, then it's not really a bug is it.

redthrowaway• 15 years ago

Unless we're talking about adding the debate to the queue of debates we're having, that is. Even then, I suppose, it should be enqueue...

Carlfish2• 15 years ago

I believe this is the tool you are looking for.

http://findbugs.sourceforge.net/bugDescriptions.html#RV_01_T...

nradov• 15 years ago

I'm amazed to still find Java programmers who don't run Findbugs on all their code. It's free, fast, and has a low rate of false positives.

erez• 15 years ago

"queue debate about tools that hold your hands vs understanding what you are actually writing"

For a start, explain why, in Java, (int) Math.random() is a bug. A programmer that happens upon this thread, now knows "this is bad", but has no clue why. A tool will only make the programmer even more reliant on tools and IDEs, so this kind of bug will be eliminated, but the frame of mind that spawned it will live on.

Carlfish2• 15 years ago

Findbugs, the popular Java tool I linked to in another comment, explains the bug like this:

"A random value from 0 to 1 is being coerced to the integer value 0. You probably want to multiple the random value by something else before coercing it to an integer, or use the Random.nextInt(n) method."

That's about as good an explanation as you'd get from anyone.

The way I see it is this. Simple bugs like this happen because _human beings are flawed._ All it takes is a momentary lapse of concentration and you've put the cast in the wrong place in a method you don't write a test for because you're in a hurry, and all it's doing is generating a random number with a standard API so why bother? (Or it's something you wouldn't normally even test for, like assuming something is re-entrant when it isn't)

We're inevitably going to make a certain number of mistakes a day, and it's our duty to put systems into place that catch those mistakes before they cause any more damage than they should.

rmccue• 15 years ago

For those who haven't picked up the error: Math.random() returns a value between 0 and 1, and the typecasting floors the value.

ghempton• 15 years ago

Majority of these results are examples and test cases which is probably why the bugs have not manifested themselves.

merloen• 15 years ago

Any other examples of bugs that you can search for?

A common bug in published code

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