C - No Prime Differences

#include <bits/stdc++.h>

using namespace std;

#define ll     long long
#define _test   int _TEST; cin>>_TEST; while(_TEST--)
#define pb     push_back
#define ppb    pop_back

int main()
{
    _test
    {
        int n, m;
        cin>>n>>m;

        vector<vector<int>> a(n, vector<int> (m, 0));

        if(n == 4)
        {
            for(int i=1; i<=n; i++)
            {
                for(int j=1; j<=m; j++)
                    a[i-1][j-1] = i + (j-1)*4;
            }

            for(int i=0; i<n; i++)
            {
                for(int j=0; j<m; j++)
                    cout<<a[i][j]<<" ";
                cout<<"\n";
            }
            continue;
        }
        else
        {
            int x = 1;
            for(int i=0; i<n; i++)
            {
                for(int j=0; j<m; j++)
                    a[i][j] = x++;
            }
        }

        int x = 0;
        for(int i=0; i<n; i+=2, x++)
        {
            for(int j=0; j<m; j++)
                cout<<a[i][j]<<" ";
            cout<<"\n";
        }

        for(int i=1; i<n; i+=2, x++)
        {
            for(int j=0; j<m; j++)
                cout<<a[i][j]<<" ";
            cout<<"\n";
        }
    }
}