Initializing array in cpp and padding with zeros

I'm new a c++, switched from matlab to run simulations faster.
I want to initialize an array and have it padded with zeros.

    # include <iostream>
# include <string>
# include <cmath>
using namespace std;

int main()
{
    int nSteps = 10000;
    int nReal = 10;
    double H[nSteps*nReal];
    return 0;
}

It produces an error:

expected constant expression    
cannot allocate an array of constant size 0    
'H' : unknown size

How do you do this simple thing? Is there a library with a command such as in matlab:

zeros(n);

Solution

Stack-based arrays with a single intializer are zero-filled until their end, but you need to make the array bounds equal to be const.

#include <iostream>

int main()
{       
    const int nSteps = 10;
    const int nReal = 1;
    const int N = nSteps * nReal;
    double H[N] = { 0.0 };
    for (int i = 0; i < N; ++i)
        std::cout << H[i];
}

Live Example

For dynamically allocated arrays, you best use std::vector, which also does not require compile-time known bounds

#include <iostream>
#include <vector>

int main()
{
    int nSteps = 10;
    int nReal = 1;
    int N = nSteps * nReal;
    std::vector<double> H(N);
    for (int i = 0; i < N; ++i)
        std::cout << H[i];
}

Live Example.

Alternatively (but not recommended), you can manually allocate a zero-filled array like

double* H = new double[nSteps*nReal](); // without the () there is no zero-initialization