How to read integers line by line from a text file in c++?

Question

So I have a problem about a graph and I have to read from an input file multiple cases and check a propriety. Each case has on the first line n, the number of nodes it has, and on the second line a sequence of integers x y, where (x, y) is an edge. The problem is that I don't know how many edges I have, so I don't know where to stop reading.

Example:

Input File :

5

1 2 1 3 1 4 2 5 3 5 4 5

7

1 2 4 5 2 6

I looked everywhere on the web for a solution, but couldn't find one that works for me. Most of the solutions I found only read strings. I tried to find and stop at '\n' but that didn't worked at all. Int('\n') is 10 on my computer, so it got confused with the 10s from my sequence of edges. And it didn't even read the '\n'.

You need to improve your searching techniques. I answered a similar question last week. — Thomas Matthews, Feb 24 '20 at 20:15
tip: you do not have to tell us that you didnt find a solution elsewhere, why would you ask here if you did? Writing "I looked everywhere on the web for a solution, but couldn't find one" conveys no useful information but rather makes us suspect that you just didnt search enough — 463035818_is_not_an_ai, Feb 24 '20 at 20:23

score 0 · Answer 1 · answered Feb 24 '20 at 20:24

Here is a solution:

struct Edge
{
  int x;
  int y;
  friend std::istream& operator>>(std::istream& input, Edge& e);
};

std::istream& operator>>(std::istream& input, Edge& e)
{
  input >> e.x;
  input >> e.y;
  return input;
}

Here's some main code:

int node_quantity = 0;
std::vector<Edge> database;
std::cin >> node_quantity;

// Ignore the newline following the number.
std::cin.ignore(1000, '\n');

// Start at the beginning of the line.
std::string text_line;
std::getline(std::cin, text_line);
// Read in the edges
std::istringstream text_stream(text_line);
Edge e;
while (text_stream >> e)
{
    database.push_back(e);
}

The above code creates an edge structure and overloads operator>> for reading in an edge.

The second code fragment reads in a line of edges and uses a std::istringstream to read all the edges in the text line.

How to read integers line by line from a text file in c++?

1 Answers1