Cannot access element of 2d heap vector using operator[]

Question

I have a 2d vector that needs to be allocated on the heap and am using the below line of code to declare and size it.

vector<vector<double>> *myArray = new vector<vector<double>>(x, vector<double>(y));

where x and y are the number of rows and columns respectively.

When I try to access an element of the vector using myArray[0][0] = 3.0;, I get the following error,

error: no viable overloaded '='

myArray[0][0] = 3.0;

I would appreciate any help in figuring this out.

Notes:

1. The number of rows and columns needs to be dynamic, hence myArray is on the heap.

2. The array needs to be resizable, which is why I am using std::vector.

3. I understand that I can create a vector of vectors (number of rows) and then in a for-loop resize each row element to the required number of columns. What I do not understand is why the above code does not work since as far as I know it should perform the same function.

Answer 1

For some strange invalid reason, you are using a pointer to a vector. Since operator[] works with pointers, when you do

myArray[0][0] = 3.0;

you are actually accessing a vector<double>, not a double, because myArray[0] gets you a vector<vector<double>>.

The obvious fix is not to use a pointer in the first place:

vector<vector<double>> myArray(x, vector<double>(y));

Answer 2

must be:

(*dataArray)[0][0] = 3.0