I want to do something like:
int a[][]; // I know this code won't work, its to demonstrate what I want to do
void func(int n, int m){
a = int[n][m];
}
that is, initialise a global array whose size depends on function input. If this array was local, it would be a trivial case, but I don't know how to do this in the case shown above. Any help would be very useful!
You can create a matrix with std::vector:
std::vector<std::vector<int>> a;
void func(int n, int m) {
a.resize(n);
for(int i = 0; i < n; i++) {
a[i].resize(m);
}
}
Then you can access elements in the same way you do with int a[][]:
a[i][j] = number;
One way to achieve this is to encapsulate a flat std::vector in a Matrix class and use math to get an element with row and column as in this example:
template<typename T>
class Matrix {
private:
vector<T> vec;
//...
public:
T& get_value(size_t const row, size_t const col) {
return vec[row * col_count + col];
}
};
you can try this
int ** a; // is a pointer of two dimension
void func(int n, int m){
a = new int*[n]; //dynamic allocation global pointer a
for(int i = 0; i < n; i++)
a[i] = new int[m]();
}
new has no place here. It is dangerous and error prone and a lot worse than std::vector<std::vector<int>> a; void func(int n, int m){ a = std::vector<std::vector<int>>(n, std::vector<int>(m)); }
std::vectorunder the hood. That gives you the best performance and ease of use.std::vectorto handle that for you.aa pointer into an array of pointers instead of an array of arrays. But initializing it and cleaning it up get a bit tricky, especially considering exception safety.