Pair of numbers using vector

[1vf9z8AR]

This piece of code is not using the stl pair but vector of vector.
I am not able to understand how it is taking a pair as input that too with only one "cin"

K is the number of pairs to be input

      vector<vector<int>> obstacles(k);
      for (int i = 0; i < k; i++) {
        obstacles[i].resize(2);

        for (int j = 0; j < 2; j++) {
            cin >> obstacles[i][j];
        }

        cin.ignore(numeric_limits<streamsize>::max(), '\n');
    }

[Duthomhas]

It is in a loop, inside another loop.

A good way to get through code you don’t expect is to trace it (I prefer manually, with pen and paper, or a good text editor):

k = 3 (or whatever, but three is a good choice here)
i = 0
j = 0
cin >> obstacles[0][0]
j = 1
cin >> obstacles[0][1]
j = 2, done
cin.ignore()

i = 1
j = 0
cin >> obstacles[1][0]
j = 1
cin >> obstacles[1][1]
j = 2, done
cin.ignore()

i = 2
j = 0
cin >> obstacles[2][0]
j = 1
cin >> obstacles[2][1]
j = 2, done
cin.ignore()

i = 3 == k, done

With that, you can follow along how it reads something like

 12 7
19 -4
52 13

or even

 12 7  // my cool list of numbers!
19 -4
52 13

Hope this helps.

[salem c]

> I am not able to understand how it is taking a pair as input that too with only one "cin"
You have a cin inside a for loop which iterates twice.
How is that not a 'pair' of values.

[keskiverto]

You do the 'resize' relatively late.
You could initialize the vector with value.
You do know that size is two, as in pair. So do array (and tuple).

vector<vector<int>> obstacles( k ); // k empty vectors (of int)
vector<vector<int>> obstacles( k, vector<int>( 2 ) ); // k two-element vectors
vector<array<int,2>> obstacles( k ); // k two-element arrays 

Can you trust that the user gives valid data?
No. Take what you can, but no half-baked entries:

vector<array<int,2>> obstacles;
obstacles.reserve( k ); // still empty
int lhs = 0;
int rhs = 0;
while ( obstacles.size() < k && cin >> lhs >> rhs )
{
  obstacles.push_back( {lhs, rhs} );
  cin.ignore( numeric_limits<streamsize>::max(), '\n' );
}

[1vf9z8AR]

thanks. cool way to input pair.

[AbstractionAnon]

When you said a pair of numbers, I automatically thought of std::pair.

vector<pair<int, int>>   obstacles(k);

   int lhs, rhs;
   while (obstacles.size() < k && cin >> lhs >> rhs)
       obstacles.push_back( make_pair(lhs, rhs) ); 

The advantage of std::pair is that lhs and rhs do not have to be the same type.