initializing 2d array with default value in cpp

Question

I want to create a N*N array with default value -1 in cpp, I tried many ways like memset, fill, fill_n but non of them worked...

here's my code:

    N = 10;
    int **m = new int*[N];
    for(int i = 0; i < N; ++i) {
        int a[N];
        fill_n(a, N, -1);
        for(int j = 0; j < N; j++)
            cout << a[j] << endl; // values here are all -1
        m[i] = a;
        // m[i] = new int[N];
    }
    // memset(m, -1, sizeof(m));
    
    for(int i=0; i < N; i++) {
        for(int j = 0; j < N; j++) {
            // value output here are wrong, looks like random numbers
            cout << "[" << i << "][" << j << "]" << m[i][j] << endl; 
        }
    }

output value are

[0][0]7
[0][1]32625
[0][2]2045453384
[0][3]21958
[0][4]-279530816
[0][5]32767
[0][6]-1010169277
[0][7]32625
[0][8]-279532512
[0][9]32767
[0][10]2036459499
[0][11]21958
[1][0]-1009461504
[1][1]32625
[1][2]2045453384
[1][3]21958
[1][4]-279530816
[1][5]32767
[1][6]-1010169277
[1][7]32625
...

In contrast, when I assign with two for loops, it can work correctly...

    for(int i = 0; i < N; i ++)
        for(int j = 0; j < N; j++)
            m[i][j] = -1;
    
    for(int i=0; i < N; i++) {
        int *p = m[i];
        for(int j = 0; j < N; j++) {
            int a = p[j];
            cout << "[" << i << "][" << j << "]" << a << endl;
        }
    }

However, N ranges from 10 to 100,000, so I can't use forloop to construct such huge array...

what did I do wrong?

int a[N]; is not legal C++ code to begin with. Then you do m[i] = a; right before a goes out of scope, leaving m[i] dangling. — n. m. could be an AI
– n. m. could be an AI, Commented May 8, 2021 at 18:03

MatG · Accepted Answer · 2021-05-10 11:27:13Z

Since you tagged the question as c++, it's advisable to take advantage of the standard library; when you need dynamically allocated arrays you normally use the std::vector class:

//https://gcc.godbolt.org/z/6n9c1PYvx
#include <iostream>
#include <vector>

int main()
{
    std::size_t N = 2;
    std::vector< std::vector<int> > v(N, std::vector<int>(N,-1) );
    for(std::size_t i=0; i<v.size(); ++i)
       {
        for(std::size_t j=0; j<v[i].size(); ++j)
           {
            std::cout << "v[" << i << "][" << j <<"] = " << v[i][j] << '\n';
           }
       }
}

This has N+1 dynamic allocations. Since this is a burdensome operation, it is advisable to do it as little as possible. Dealing with multidimensional arrays it's usually more efficient to allocate a single big array and accessing its elements appropriately like this:

int main()
{
    std::size_t N = 2;
    std::vector<int> v(N*N, -1);
    for(std::size_t i=0; i<N; ++i)
       {
        for(std::size_t j=0; j<N; ++j)
           {
            std::cout << "v[" << i << "][" << j <<"] = " << v[i+N*j] << '\n';
           }
       }
}

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