2D vector of unknown size

Question

I defined an empty vector of vectors :

vector< vector<int> > v;

How do i fill that empty vector with vector of size 2 integers ( from input ) each iteration of while loop?

while ( cin >> x >> y ) {
  //....
}

Will this one work? Or what's the best and most elegant / effective way of doing it?

while ( cin >> x >> y )
{
   vector<int> row;
   row.push_back( x );
   row.push_back( y );
   v.push_back( row );
}

Answer 1

As pointed out by JerryCoffin, you probably better use a :

struct Point {
    int x;
    int y;
};

and then you might overload the output operator

std::ostream& operator<< (std::ostream& o,const Point& xy){
    o << xy.x << " " << xy.y;
    return o;
}

and similar the input operator (see eg here ). And then you can use it like this:

int main() {
    Point xy;
    std::vector<Point> v;
    v.push_back(xy);
    std::cout << v[0] << std::endl;
    return 0;
}

Answer 2

The another way of doing so is to push the value in a 1-D vector and then push that 1-D vector in 2-D vector. I have also printed the values as a test.

#include<bits/stdc++.h>
using namespace std;
int main()
{
    int t,n,temp;
    cin>>t;
    vector<vector<int>> value;
    for(int i=0;i<t;i++)
    {
        cin>>n;
        vector<int> x;
        for(int j=0;j<n;j++)
        {
            cin>>temp;
            x.push_back(temp);
        }
        value.push_back(x);
    }
    for(int i=0;i<t;i++)
    {
        for(int j=0;j<value[i].size();j++)
        {
            cout<<value[i][j]<<" ";
        }
        cout<<endl;
    }
    return 0;
}

Hope it helps!!

Answer 3

I wrote it - compilator didnt say anything but i have never used vectors so i haven't figured out how to print it yet

You can print using for example a range based for loop (C++11):

for (const auto &vec : v) {     // for every vector in v
    for (const auto &num : vec) // print the numbers
        cout << num << " ";
    cout << '\n';
}

And regular for loop:

for (unsigned int i = 0; i != v.size(); ++i) {
    for (unsigned int j = 0; j != v[i].size(); ++j) {
        cout << v[i][j] << " ";   
    }
    cout << '\n';
}