How to call a dynamic matrix into a function? - c++

Question

I have declared a matrix this way:
double **MB;
MB = new double *[650000];
for (int count = 0; count < 650000; count++)
MB[count] = new double [2];
Now I want to call my matrix in a function that should modify it.
bool notindic (..., double MB [][2], ...) {}
and in the main:
notindic(..., MB, ...)
Now it gives me this error: *[Error] cannot convert 'double**' to 'double ()[2]' for argument '3' to 'bool notindic(std::string, std::string, double ()[2], int, int)'
How can I fix it?
Thank you.

Answer 1

Just pass array pointer as parameter
#include <iostream>
const int NX = 65;
const int NY = 2;
bool notindic(double** MB) {
for(int i = 0; i < NX; ++i) {
for(int j = 0; j < NY; ++j) {
MB[i][j] = i + j;
}
}
}
int main() {
double **MB = new double *[650000];
for (int count = 0; count < 650000; count++) {
MB[count] = new double [2];
}
notindic(MB);
for(int i = 0; i < NX; ++i) {
for(int j = 0; j < NY; ++j) {
std::cout << MB[i][j] << " ";
}
std::cout << std::endl;
}
}

Answer 2

Forget about all that pointer nonsense. It's error-prone, exception-unsafe, hard to write, hard to read, hard to maintain and may perform poorly. Represent your matrix as std::vector<double> and calculate the offsets accordingly.
bool notindic (std::vector<double> const& matrix, int m) {
// ...
auto const element = matrix[y * m + x];
// ...
}
auto const m = 650000;
auto const n = 2;
std::vector<double> my_matrix(m * n);
auto const result = notindic(my_matrix, m);
While you're at it, wrap it in a class like this:
template <class T>
class Matrix final
{
public:
Matrix(int m, int n) :
data(m * n),
m(m)
{
}
T& operator()(int x, int y)
{
return data[y * m + x];
}
T const& operator()(int x, int y) const
{
return data[y * m + x];
}
private:
std::vector<T> data;
int m;
};
bool notindic (Matrix<double> const& matrix) {
// ...
auto const element = matrix(x, y);
// ...
}
auto const m = 650000;
auto const n = 2;
Matrix<double> my_matrix(m, n);
auto const result = notindic(my_matrix);
Add additional member functions if you need them.