dynamically allocating memory to matrices - c++

Question

I am trying to convert an array to a matrix, so I dynamically allocated memory to the matrix, but I'm getting an error:
CRT detected that the application wrote to memory after end of heap buffer
int main() {
float a[9] = { 1, 3, 5, 6,4,6,5,6,8};
int b = sizeof(a)/sizeof(a[0]);
int r = sqrt(b) - 1;
float **A_mat = new float*[r];
//A_mat = ;
for (int i = 0; i <= r; i++)
A_mat[i] = new float[r];
for (int j = 0; j <= r; j++) {
for (int i = 0; i <= r; i++) {
A_mat[j][i] = a[i + j * (r+1)];
}
}
cout << "a[0,0] is " << A_mat[0][0] << endl;
cout << "a[0,2] is " << A_mat[0][2] << endl;
cout << "a[1,0] is " << A_mat[1][0] << endl;
cout << "a[2,0] is " << A_mat[2][0] << endl;
for (int i = 0; i <= r; i++) {
delete[] A_mat[i];
}
delete[] A_mat;
system("pause");
}

Answer 1

Your code has two major issues:
Issue 1:
float **A_mat = new float*[r]; // allocating memory for 2 rows because r = sqrt(9) - 1 => r = 2
for (int i = 0; i <= r; i++)
A_mat[i] = new float[r]; // Usage of A_mat[r] for r = 2 is out of bounds. Also, memory is allocated for 2 columns.
Issue 2:
for (int j = 0; j <= r; j++) {
for (int i = 0; i <= r; i++) {
A_mat[j][i] = a[i + j * (r+1)]; // accessing indices (0, 2), (1, 2), (2, 0), (2, 1), (2, 2) are out of bounds
}
}
Instead, you should allocate (r + 1) size memory. Like this:
int main() {
float a[9] = { 1, 3, 5, 6,4,6,5,6,8};
int b = sizeof(a)/sizeof(a[0]);
int r = sqrt(b) - 1;
float **A_mat = new float*[r + 1];
//A_mat = ;
for (int i = 0; i <= r; i++)
A_mat[i] = new float[r + 1];
for (int j = 0; j <= r; j++) {
for (int i = 0; i <= r; i++) {
A_mat[j][i] = a[i + j * (r+1)];
}
}
cout << "a[0,0] is " << A_mat[0][0] << endl;
cout << "a[0,2] is " << A_mat[0][2] << endl;
cout << "a[1,0] is " << A_mat[1][0] << endl;
cout << "a[2,0] is " << A_mat[2][0] << endl;
for (int i = 0; i <= r; i++) {
delete[] A_mat[i];
}
delete[] A_mat;
system("pause");
}
Suggestions:
1) Never use raw pointers. Always look for safer alternatives. In this case, you should go for std::vector.
2) Never use naked new. It is generally the major source of Memory leaks. Though currently, its not the case for you. But it might become an issue in the future.
3) Always remember that array starts from 0-indexing in C++.

Answer 2

You have problem with for loop. The index is from 0 to (r-1). But your for loop from 0 to r. It made out of bound of your array.
1> So please replace this for loop
for (int i = 0; i <= r; i++)
by
for (int i = 0; i < r; i++)
2> And replace this statements
for (int j = 0; j <= r; j++) {
for (int i = 0; i <= r; i++) {
A_mat[j][i] = a[i + j * (r+1)];
}
}
by
for (int j = 0; j < r; j++) {
for (int i = 0; i < r; i++) {
A_mat[j][i] = a[i + j * r];
}
}