Read and Write File Line by Line in C++

Introduction

In this lab, you will learn how to read and write a file line by line using C++ programming language. You will create a file, write to it, and then read its content one line at a time.

Creating and opening a file

Create a C++ file named file_io.cpp in the ~/project directory. In this step, you will create and open a file named example.txt.

#include <fstream>

int main() {
    // Creating and opening a file
    std::ofstream ofs;
    ofs.open("example.txt", std::ofstream::out | std::ofstream::trunc);
    ofs.close();
    return 0;
}

Explanation:

• ofstream is a class for writing to files.
• ofs is an object of ofstream class.
• open() method is used to create and open a file.
• In the open() method, the first argument is the name of the file to be created and the second argument specifies the mode in which the file should be opened. Here, out specifies that the file is opened for writing, and trunc specifies that the file should be truncated if it already exists.

Writing to a file

In this step, you will write to the file example.txt.

#include <fstream>
#include <iostream>
#include <string>

int main() {
    std::ofstream ofs;
    ofs.open("example.txt", std::ofstream::out | std::ofstream::trunc);
    if (ofs.is_open()) {
        std::cout << "Enter text to write to the file (Enter empty line to exit):" << std::endl;
        std::string line;
        do {
            std::getline(std::cin, line);
            if (line.length()) {
                ofs << line << std::endl;
            }
        } while (line.length());
        ofs.close();
    } else {
        std::cerr << "Error in opening the file" << std::endl;
    }
    return 0;
}

Explanation:

• std::cout is used to display a message on the console asking the user to enter text to write to the file.
• std::getline() is used to read the user input line by line and store it in the line variable.
• if (line.length()) is used to check whether the user has entered any text or not. If the user has not entered any text, then the loop will exit.
• ofs << line << std::endl; is used to write the user input to the file followed by a newline character.
• std::cerr is used to display an error message on the console if the file cannot be opened.

To run this program, go to the ~/project directory and run the following command:

g++ file_io.cpp -o file_io && ./file_io

This program will prompt the user to enter text to write to the file, and then write the user input to the file.

Reading from a file

In this step, you will read the contents of the example.txt file.

#include <fstream>
#include <iostream>
#include <string>

int main() {
    std::ifstream ifs;
    ifs.open("example.txt");
    if (ifs.is_open()) {
        std::cout << "Contents of the file:" << std::endl;
        std::string line;
        while (std::getline(ifs, line)) {
            std::cout << line << std::endl;
        }
        ifs.close();
    } else {
        std::cerr << "Error in opening the file" << std::endl;
    }
    return 0;
}

Explanation:

• std::ifstream is a class for reading from files.
• ifs is an object of ifstream class.
• open() method is used to open an existing file.
• In the open() method, the argument is the name of the file to be opened.
• std::cout is used to display a message on the console to indicate that the contents of the file are being displayed.
• std::getline() is used to read the contents of the file line by line and store it in the line variable.
• std::cout << line << std::endl; is used to display the contents of the file line by line on the console.

To run this program, go to the ~/project directory and run the following command:

g++ file_io.cpp -o file_io && ./file_io

This program will display the contents of the example.txt file on the console.

Summary

In this lab, you learned how to create, write to, and read from a file using C++ programming language. You learned how to use ofstream and ifstream classes to write to and read from files, respectively. You also learned how to use open() method to create and open a file, and getline() method to read the contents of a file line by line.