How to work on a sub-matrix in a matrix by pointer?

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You cannot dereference mat+1 and reinterpret that as a pointer to a whole matrix. Instead provide the offsets as arguments to your function (I assume n-by-n square matrices):

void rotate(int** mat, size_t i, size_t j, size_t n) {
    // assuming row-wise storage
    int *row0 = mat[j];     // assumes j < n
    int *row1 = mat[j + 1]; // assumes j + 1 < n
    // access row0[i..] and row1[i..]
}

If you had continuous storage for your matrix, you could do the following instead:

rotate(int* mat, size_t i, size_t j, size_t n) {
    int atIJ = mat[j * n + i]; // assuming row-wise storage
    // ...
}

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