Java: Maths in the default libraries

Civilian_Target

Does anyone know if Sun are planning to build in some slightly more advanced maths functionality into their defualt VM set or is the Math class all we're going to get.

I was looking for some of the functionality of my calculator (specificly factorial and nCr) for use in a java program I was writing today and was quite surprised not to find it in the API docs, and a quick google turned up no useful information.

Maybe I should be writing this program in Fortran!

Hobbes

Short answer: Nope. Maths package is what you get.

A quick google appears to have code for nCr and factorials.

Civilian_Target

Well - yeah, thats exactly what I did, factorial's just 3 lines, and one you have that nCr's only another 2 but it's not as handy as saying Math.fac...

Mutant_Fruit

public int Factorial(int number)
{
     return abs(number)>0 ? number*Factorial(abs(number)-1) : 1;
}

public int NCR(int n, int r)
{
    return Factorial(n) / Factorial(r);
}

Not the most efficient code, but bloody short

Civilian_Target

Pfft - yeah wrote it myself in the end too, except I used small letters for the method names! Only needed r = 2, but also needed to calculate triangular numbers... Incidentally, if you were writing NCR, it's actually fac(n)/(fac(r)*fac(n-r))
I have to do a heavier version next week, and it's gonna be in Fortran or I might take the time to learn Matlab.

public int tn(int i){
		int r = 0;
		for(int j = 0; j < i; j++)
			for(int k = j+1; k < i; k++)
				r++;
		return r;
	}
	public int fac(int i){
		if(i <=1 ) return 1;
		return i * fac(i - 1);
	}
	public int n2r(int i){
		return fac(i) / fac (i - 2);
	}

verbatim

Whilst lecturers love to teach recursion in Java, its not the best thing to use in the realworld. If you have too many levels of recursion in Java, your stack overflows, but since your code appears syntactically perfect it can take a while to figure out where the error is.

The Maths support in Java has most of what you need to get by. What really annoys me is the BigInteger and BigDecimal classes, which only have a tiny fraction of the methods that the Maths class has.

Civilian_Target

Nonsense. If used sensibly recursion works very well in the real world, and creates simple efficient code. If used badly it creates unnecessary n-p hard problems

At the end of the day, most for and while loops behave exactly the same way as recursive statements in machine code so nothing is really lost by using recursion except a few ugly lines of for loop!

Granted, my n2r method is not efficient because it calculates fac(i) and fac(i-2) whereas using a loop, fac(i-2) could be carried if you wrote it well. But I needed a quick fix, so that's what I did.

I find the Java maths library to be woefully under-resourced, but I need the code portability. Otherwise I'd write it in Fortran.

Mutant_Fruit

verbatim wrote:

Whilst lecturers love to teach recursion in Java, its not the best thing to use in the realworld. If you have too many levels of recursion in Java, your stack overflows, but since your code appears syntactically perfect it can take a while to figure out where the error is.

I think a stack overflow exception would be pretty obvious to figure out (im sure java throws one of those type of exceptions when the stack overflows). There are places where recursion is *much* better than simple looping. Supposing i wanted you to do a search on your harddrive and find all the files that contain the word "test" in the filename. Recursion would be the easiest way to do that.

FindAllFolders(string PATH, ArrayList results)
{
     foreach(Folder newFolder in the current PATH)
          FindAllFolders(newFolder, results);

     foreach(File f in all the files in this PATH)
         if(file.Name.Contains("test"))
              results.Add(f);
}

Now, how would you do that without recursion? (note that i used psuedo code, actual code would be ~4-6 lines longer)

Civilian_Target

For those of you that do need more Maths functionality: I've started using Jama: http://math.nist.gov/javanumerics/jama/

It's matrix features are particularly swish.