How to handle matrices in java

[EDIT: 20110718 13:41 CDT - reformat - WDR]

I trying to call matlab function from java that takes two matrices as arguments.

I have to read the data file using java and after I have constructed the matrices (2 x 255 and 1 x 255) I call the matlab function compiled into java.

matlab:

data=load(L);
x = [data(:,1) data(:,2)];
y = data(:,3);

gp=gp_optim(gp,x,y); % gp is a structure

java: 
data = main.readFile();
double[][] x = new double[255][2];
double[][] y = new double[255][1];

for (int i = 0; i < 255; i++) {
x[i][0] = data[i][0]; 
x[i][1] = data[i][1];
y[i][0] = data[i][2];
}

gpObj = li.gp_optim(1, gpObj[0], x, y);

Error:

com.mathworks.toolbox.javabuilder.MWException: Error using ==> chol
Matrix must be positive definite.
  at com.mathworks.toolbox.javabuilder.internal.MWMCR.mclFeval(Native Method)
  at com.mathworks.toolbox.javabuilder.internal.MWMCR.access$600(MWMCR.java:23)
  at com.mathworks.toolbox.javabuilder.internal.MWMCR$6.mclFeval(MWMCR.java:902)
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:616)
  at com.mathworks.toolbox.javabuilder.internal.MWMCR$5.invoke(MWMCR.java:800)
  at $Proxy0.mclFeval(Unknown Source)
  at com.mathworks.toolbox.javabuilder.internal.MWMCR.invoke(MWMCR.java:475)
  at gpStuff.GpStuff.gp_optim(GpStuff.java:557)
  at main.Main.main(Main.java:132)

So it seems that I don't call the matlab function correctly. How the matrices should be handled in java?

Answer 1

[Friedrich]

Hi,

the BUILDER JA comes with a javabuilder.jar (located in C:\Program Files\MATLAB\R2011a\toolbox\javabuilder\jar) which provides MATLAB datatypes, MWARRAY ( <- abstract class). Please look here for the example how to handle matrices:

http://www.mathworks.com/help/releases/R2011a/toolbox/javabuilder/ug/bsqrikz-11.html

Click on the getfactor.java to see the java code. For the MWARRAY class definition see here:

http://www.mathworks.com/help/releases/R2011a/toolbox/javabuilder/ug/bqp6omw-37.html

[Jani]

Thanks! that I were looking for.

Answer 2

[Titus Edelhofer]

Hi,

is this the MATLAB code?

data=load(L); 
x = [data(:,1) data(:,2)]; 
y = data(:,3);

This will not work, since data will be a structure... But besides this: one easy way to find out what's going on: put into the first line of your MATLAB code some

if isdeployed 
  save c:\temp\inputdata.mat
end

Then compile and run, and in MATLAB you can do:

load inputdata
% call your function in debug mode ...

Titus

[Walter Roberson]

data will not be a structure if what is being load()'d is a text file. As the documentation indicates,

S = load(filename) loads the variables from a MAT-file into a structure array, or data from an ASCII file into a double-precision array.

[Titus Edelhofer]

Hi Walter,

yes of course, it could be an ascii file, didn't think of this ...

Titus

Answer 3

[Jani]

Thanks for your responses. Matlab loads the data as Walter mentioned, but the problem is with java. I don't know what kind of structure the matrix and vector should be. I have tested couple of different types build-in java, but nothing works. Could it be some matlab class (in javabuilder package) that corresponds to matlab matrix?