How to input an matrix style txt file instead of defining my own int 2D array for C++

So I'm pretty new to C++ but i think im gettting the hang of it a bit.

As part of an excersize, I have to take an input text file and apply this in a "shortest distance algorithm" where ultimatly I want to output all the shortest distances and routes but i haven't gotten that far yet. I have used the Floyd Warshall algorithm. For now my question is, how do i replace a self written int array by a text input. the input array is just numbers but actually represents distances between nodes. The test array that im using now only has 3 nodes, but i want to be able to expand it to a much larger node amout, say 100.

example test matrix:

0 1234567 100

1234567 0 400

100 400 0

Should be read as:

           node1      node2       node3
  node 1    0          999999     100
  node 2   999999       0         400
  node 3    100        400         0 

The large numbers: 999999 represents a distance that is too large too count as a edge.

As of now my code looks something like this:

#include<stdio.h>

// Number of vertices
#define V 3

// Define 999999 as a distance that is too large to represent a edge connection
#define TooLarge 999999

// The print function
void printSolution(int dist[][V]);

        // Distance algorithm
    void Distance (int distgraph[][V])
    {
        // output matrix that will have the shortest distance for every vertice
        int dist[V][V], i, j, k;

        // initial values for shortest distance are based on shortest paths.
        for (i = 0; i < V; i++)
            for (j = 0; j < V; j++)
                dist[i][j] = distgraph[i][j];

        // Add all vertices to the set of intermediate vertices.
             for (k = 0; k < V; k++)
        {
            // use all vertices as seperate source
            for (i = 0; i < V; i++)
        {
            // use all vertices as destination for the earlier determined source

            for (j = 0; j < V; j++)
            {
                // If vertex k is on the shortest path from i to j, then update the value of dist[i][j]
                if (dist[i][k] + dist[k][j] < dist[i][j])
                    dist[i][j] = dist[i][k] + dist[k][j];
            }
        }
    }

    // Print the shortest distance matrix
    printSolution(dist);
}

// The print function
void printSolution(int dist[][V])
{
    printf ("Shortest distance matrix \n");
    for (int i = 0; i < V; i++)
    {
        for (int j = 0; j < V; j++)
        {
            if (dist[i][j] == 999999)
                printf("%7s", "TooLarge");
            else
                printf ("%7d", dist[i][j]);
        }
        printf("\n");
    }
}

// driver program to test above function
int main()
{
      int distgraph[V][V] = { {0, 1234567, 100},
                              {1234567, 0, 400},
                              {100, 400, 0,},
                            };

    // Print the solution
    Distance(distgraph);
    return 0;
}

Hopefully someone can help me, I have the feeling im just forgetting something stupid. I have tried to inport the textfile using this type of code:

    using namespace std;

double distances [3][3];

int main () {
  int x, y;
  ifstream in("citytest.txt");

  if (!in) {
    cout << "Cannot open file.\n";
    return 0;
  }

  for (y = 0; y < 3; y++) {
    for (x = 0; x < 3; x++) {
      in >> distances[x][y];
    }
  }
    cout << distances[3][3] << " " << endl;
      in.close();

Which i know works, but only inports a predetermind part of the matrix whereas i want to input the entire array. (the cout function is just there to test if the correct distances were given as an input)