Question

hello i have to assign a matrix to a double pointer i wrote this code for simplify the things.

int main (int argc, const char * argv[]) 
{

int ** matrix;
//// initialize the matrix 
matrix = malloc(sizeof(int *)*3);

for (int i=0; i<3; i++)
{

    matrix[i] = malloc(sizeof(int)*3);

}

// initialize the 2-d array
int matrixx[][3] = { {1,3,4},{5,3,1},{24,5,1} };

for (int i=0; i<3; i++) {
    ///good kind of assignment 
    matrix[i] = &matrixx[i];
}

/// error how i fix it ?
matrix = &matrixx[0][0];
}

Answer 1

You can’t fix it. The type of matrix is int **. The type of &matrixx[0][0] is int *. These are incompatible.

Your “good kind of assignment” is the correct approach if you want to maintain the array-of-pointers mechanism. However, you have the syntax for it slightly wrong; it should be:

for (int i=0; i<3; i++) {
    ///good kind of assignment 
    matrix[i] = matrixx[i];
}

Also, you don’t need the loop that does this:

for (int i=0; i<3; i++)
{
    matrix[i] = malloc(sizeof(int)*3);
}

(You’ll incur a memory leak as @Chris points out below as soon as you re-assign these pointers.) Unless, of course, your intention is to copy the contents of the matrix. In which case, your approach is definitely wrong.

[Note also that you’re not calling free() anywhere, which means you have a memory leak.]