Question

I need to get very basic input from an external file in C++. I tried searching the internet a few times but nothing really applied to what I need. This would be a .txt file that the input it coming from, and it would be filled with lines like this:

131
241
371
481

I have code already to manually get this input, and it looks like this:

using namespace std;
//Gets the initial values from the user.
    int control=0;
    while (rowb!=0){
        cout << "Row: ";
        cin >> rowb;
        cout << "Column: ";
        cin >> columnb;
        cout << "Number: ";
        cin >> numb;
        row[control]=rowb-1;
        column[control]=columnb-1;
        num[control]=numb;
        control++;
    }

This is part of a program that solves sudoko boards. The inputed numbers are the initial values that a sudoko board holds, and the user is inputing the row, column, and number that comes from a board.
What I need is to be able to create a .txt file with these numbers stored in rows so that I do not have to enter so many numbers. I have very little idea how to go about doing this. Mainly I’ll only be using the txt file for testing my program as I move along with adding more code to it. It takes 150+ entered numbers within my program just to get a single board, and it takes a lot of time. Any accidentally wrong entered value is also a huge problem as I have to start again. So how would I get C++ to read a text file and use those numbers as input?

Answer 1

You can open a file for input with std::ifstream from the header <fstream>, then read from it as you would from std::cin.

int main()
{
    std::ifstream input("somefile.txt");
    int a;
    input >> a;  // reads a number from somefile.txt
}

Obviously, you can use >> in a loop to read multiple numbers.