Question

I want to pass a 3d float array(float[][][]) from java to c edit it and return the array back to java
Curretly i can do this only with 1D arrays with this

jfloat* valuesjf = NULL;
float* valuesf = NULL;
void Java_Grids_Grid5_Update( JNIEnv* env,jobject thiz,jfloatArray values )
{
    valuesjf = (*env)->GetFloatArrayElements(env,values,NULL);
    valuesf = valuesjf;

    valuesf[0]=121+valuesf[0];//do some calculations

    (*env)->ReleaseFloatArrayElements(env, values, valuesjf, JNI_ABORT);
}

How do i do it with 3D array?

Answer 1

You could flatten the 3d float array in Java into a 1d array. You can then access each element of the original 3d array by generating the correct index into the 1d array. You would also need to pass the length of each dimension of the array to the native method so that the native method can also generate the correct indices into the 1d array

Java:

// The lengths of each dimension of the values array
int l, m, n;
// Instead of:
// float[][][] values = new float[l][m][n];
// Do this:
float[] values = new float[l * m * n];

// To access an entry at (x, y, z) in the array.
// Instead of:
// float v = values[z][y][x];
// Do this:
int i = (m * n * z) + (n * y) + x;
float v = values[i];

C++:

// You should not store a reference to the Java array as a global variable.
// The reference is only valid between the Get/ReleaseFloatArrayElements calls
// Since valuesf is assigned the same reference to the Java array,
// the same advice also applies to it
// jfloat* valuesjf = NULL;
// float* valuesf = NULL;

void 
Java_Grids_Grid5_Update(JNIEnv* env,
                         jobject thiz,
                         jfloatArray values,
                         jint l, jint, m, jint n) {

  jfloat* valuesjf = (*env)->GetFloatArrayElements(env,values,NULL);
  float* valuesf = valuesjf;

  int x, y, z;
  int i = (m * n * z) + (n * y) + x;
  valuesf[0]=121+valuesf[i]; //do some calculations

  (*env)->ReleaseFloatArrayElements(env, values, valuesjf, JNI_ABORT);

}