Java method for recursive matrix multiply?

问题

So basically, what I want to do is make a method that takes 2 matrices as arguments, and multiplies them. This is a school task, and Im asked to solve this by using recursive "Divide and Conquer". This is my code so far:

public class RecMult {

    public int[][] calc(int[][] a, int[][] b) {

        int n = a.length;
        int[][] c = new int[n][n];

        if (n == 1) {
            c[0][0] = a[0][0] * b[0][0];
        } else {
            int sub = a.length / 2;
            int[][] smalla11 = new int[sub][sub];
            int[][] smalla12 = new int[sub][sub];
            int[][] smalla21 = new int[sub][sub];
            int[][] smalla22 = new int[sub][sub];
            int[][] smallb11 = new int[sub][sub];
            int[][] smallb12 = new int[sub][sub];
            int[][] smallb21 = new int[sub][sub];
            int[][] smallb22 = new int[sub][sub];

            for (int i = 0; i < sub; i++) {
                for (int j = 0; j < sub; j++) {
                    smalla11[i][j] = a[i][j];
                    smalla12[sub + i][j] = a[sub + i][j];
                    smalla21[i][sub + j] = a[i][sub + j];
                    smalla22[sub + i][sub + j] = a[sub + i][sub + j];
                    smallb11[i][j] = b[i][j];
                    smallb12[sub + i][j] = b[sub + i][j];
                    smallb21[i][sub + j] = b[i][sub + j];
                    smallb22[sub + i][sub + j] = b[sub + i][sub + j];
                }
            }
            c[0][0] = calc(smalla11, smallb11);
        }
        return c;
    }
}

I did not complete the code because I encountered a problem pretty fast. I'm not able to set c[0][0] to calc(smalla11, smallb11) because it takes an int, and the calc method returns a int[][]. Im not actually sure what to do at this point. Sometimes, I would have wanted to return just a single int, but later I would want to return submatrices and at last the full matrix. Does anyone have any suggestions, either on how to "fix" this return issue, or maybe even a better idea on how to write the code for such a program?

回答1:

You're trying to store a 2 dimensional array (the results from calc(smalla11, smallb11)) into a single element c[0,0]. c has already been declared to be a 2 dimensional array. Are you intentionally trying to store the result in a single point 0,0?

Would this do what you need?

c =  calc(smalla11, smallb11);

This makes my brain hurt, but it's fun! :)

来源：https://stackoverflow.com/questions/21416512/java-method-for-recursive-matrix-multiply

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