java - Divide and conquer matrix multiplication base case + how to split matrix into 4 quarters -


i've been trying code divide , conquer matrix multiplication algorithm

there's problem when try divide matrix 4 quarters gives me error arrayoutofindexbound

  • i'm not sure if i'm right base case, can me out guys?

the problem @ double[][] a21 

    public static double[][] divideandconquer(double[][] , double[][] b, int dimension){  if (a.length == 1){     double[][] result = new double[1][1];     result[0][0]= a[0][0]*b[0][0];     return result; } else {     int m = dimension/2;     double[][] a11 = new double[m][m];     for(int = 0; < m ; i++){         (int j = 0 ; j< m ; j++)             a11[i][j]= a[i][j];     }                double[][] a21 = new double[m][m];             for(int = m; < dimension; i++){         (int j = 0 ; j< m ; j++)             a21[i][j]= a[i][j];     }      double[][] a12 = new double[m][m];             for(int = 0; < m ; i++){         (int j = m ; j< dimension ; j++)             a12[i][j]= a[i][j];     }        double[][] a22 = new double[m][m];             for(int = m; < dimension; i++){         (int j =  m; j < dimension; j++)             a21[i][j]= a[i][j];     }       double[][] b11 = new double[m][m];     for(int = 0; < m ; i++){         (int j = 0 ; j< m ; j++)             b11[i][j]= b[i][j];     }       double[][] b12 = new double[m][m];             for(int = 0; < m ; i++){         (int j = m ; j< dimension ; j++)             b12[i][j]= b[i][j];     }        double[][] b21 = new double[m][m];             for(int = m; < dimension; i++){         (int j = 0 ; j< m ; j++)             b21[i][j]= b[i][j];     }      double[][] b22 = new double[m][m];             for(int = m; < dimension; i++){         (int j =  m; j < dimension; j++)             b21[i][j]= b[i][j];     }              double[][] x1 = divideandconquer(a11,b11,m);             double[][] x2 = divideandconquer(a12,b21,m);             double[][] x3 = divideandconquer(a11,b12,m);             double[][] x4 = divideandconquer(a12,b22,m);             double[][] x5 = divideandconquer(a21,b11,m);             double[][] x6 = divideandconquer(a22,b21,m);             double[][] x7 = divideandconquer(a21,b12,m);             double[][] x8 = divideandconquer(a22,b22,m);         ..........................etc

as written, problem need subtract off array offset; e.g., 

a12[i][j]= a[i][j];

should be

a12[i][j-dimension]= a[i][j];

your bigger problem you're creating 4 new submatrices, going generate tons of garbage. once you've gotten working, think ways of doing "in-place" manipulating array indexes.

e.g., new api like

public static double[][] divideandconquer(double[][] , double[][] b, int aminindex, int amaxindex, int bminindex, bmaxindex){

and divide , conquer build subsets of min & max indices.


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