matrix blocking gives a segmentation fault

Question

I am trying to implement the Strassen algorithm in C++. I want to partition the square matrix 'hA' into 4 equal blocks.

// Initialize matrices on the host
float hA[N][N],ha11[N / 2][N / 2], ha12[N / 2][N / 2], ha21[N / 2][N / 2],
        ha22[N / 2][N / 2];
for (int i = 0; i < N; i++) {
    for (int j = 0; j < N; j++) {
        hA[i][j] = i;
        //hB[i][j] = i;
    }
}
for (int i = 0; i < N; i++) {
    for (int j = 0; j < N; j++) {
        printf("\n%d,%d\n", i, j);
        if (i < N / 2 & j < N / 2) {
            ha11[i][j] = hA[i][j];
        } else if (i < N / 2 & j >= N / 2) {
            ha12[i][j] = hA[i][j];
        } else if (i >= N / 2 & j < N / 2) {
            ha21[i][j] = hA[i][j];
        } else if (i >= N / 2 & j >= N / 2) {
            ha22[i][j] = hA[i][j];    //faulty!
        }
    }
}

I used above method for partitioning and it gets faulty as shown in the output below. But it works fine when I remove the last comparison in the 'if else' ladder.

Why does 'i' have a wrong value that is even outside the loop condition? Is there a more convenient way to do the partitioning than this way?

Answer 1

Your arrays should be N x N, not N/2 x N/2.

Your use of the bitwise operator & is unusual but works. I mistakenly thought you need a logical and ( && ) instead. Still, for readability I'd suggest the &&. You get the short circuiting with too.

    for (int i = 0; i < N; i++) {
        for (int j = 0; j < N; j++) {
            printf("\n%d,%d\n", i, j);
            if (i < N / 2 & j < N / 2) {
                ha11[i][j] = hA[i][j];
            } else if (i < N / 2 & j >= N / 2) {
                ha12[i][j] = hA[i][j];
            } else if (i >= N / 2 & j < N / 2) {
                ha21[i][j] = hA[i][j];
            } else if (i >= N / 2 & j >= N / 2) {
                ha22[i][j] = hA[i][j];    //faulty!
            }
        }
    }