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Joseph Lizier edited this page Aug 20, 2015 · 1 revision

Issues and workarounds for converting arrays between Octave and Java format

Octave-Java array conversion

Conversion between native octave array types and java arrays is not straightforward (particularly for multidimensional arrays, or int arrays).

On this page, we describe these issues, and then describe the scripts provided to perform the array conversion for you. Some links about the issues are provided as well.

Issues

One can generally call java methods which require a single dimensional double[]() array by passing in a native octave single dimensional array (e.g. see OctaveMatlabExamples-Example_1).

This does not work however for multidimensional arrays, or int arrays: octave often reports that a method is not found when one tries to call a method requiring one of these types as an argument with a simple octave array as the argument.

One could create arrays of the required Java type inside octave (e.g. jDoubleMatrix = javaArray('java.lang.Double', 2, 3);) and then convert each element in an octave array into this java array. However, a. this can be time consuming, and b. this can't be done with integer arrays anyway.

Scripts to workaround the issue

We have supplied scripts to make these conversions simple for the user, and indeed to make them as efficiently as possible.

These scripts are available at demos/octave in the svn or main distributions, and are the following files:

See example use in OctaveMatlabExamples-Example_2 for octave-to-java conversion, and OctaveMatlabExamples-Example_4 for java-to-octave conversion.

The conversion is performed using the infodynamics.utils.OctaveMatrix class (which extends the org.octave.Matrix class that is distributed and installed with the octave-java package). We extended org.octave.Matrix so that we had access to new conversion routines for int data. If you examine the source code of octaveToJavaDoubleArray.m or octaveToJavaDoubleMatrix.m you will see that we simply create a infodynamics.utils.OctaveMatrix object from the octave matrix, telling it the dimensions of that matrix, then extract the Java array object of the required type.

The reverse conversion is performed with the javaMatrixToOctave script, which uses the java_convert_matrix feature (to automatically convert org.octave.Matrix to octave native types).

In Matlab, matrices are converted back and forth to java format automatically. To ensure that your code can be used in either Matlab or Octave, you can call our conversion scripts - they will work in either Matlab or Octave: in Matlab, they simply return the matrix that was passed in.

Caveats

Note that there are still some arrays that Octave will not recognise even with this conversion, e.g. see sourceArray=(rand(100,1)>0.5)*1; in OctaveMatlabExamples - Example 1. This line contains a *1 to convert the Boolean array into something that Octave will recognise as an integer array -- without the *1 the line will not work, even if we wrap it in a octaveToJavaIntArray conversion. More information is provided by looking at the output of:

javaMethod('get', 'java.lang.reflect.Array', (rand(100,1)>0.5), 1);

which gives the error java.lang.IllegalArgumentException: Argument is not an array, whereas if one adds the *1 then this error does not occur. It seems that Octave simply does not recognise rand(100,1)>0.5) as any type of array, though it is unclear why.

Links

Some helpful links if you want to know more:

JIDT -- Java Information Dynamics Toolkit -- Joseph Lizier et al.

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