Lesson 19 · 19 of 20

Writing and reading text files · C++

Save data to a text file with std::ofstream and read it back with std::ifstream in C++, in the program's current folder.

C++ · Writing and reading text files in DMarlon Studio: the example's code and its output in the terminal

`<fstream>` brings the file streams, which work like std::cout and std::cin. `std::ofstream out("temperatures.txt")` opens the file for writing in the current folder: it creates it if it is missing and empties it if it already exists. Then you write with <<, exactly as on the console.

`close()` closes the file and saves what was written; if you forget, it closes by itself when the variable goes away at the end of the block. With `std::ios::app` as a second argument, the file is opened to append at the end, and Valencia joins the other three cities.

`std::ifstream in("temperatures.txt")` opens it for reading. It is wise to check that it opened: !in is true if it failed (the file does not exist or there is no permission), and then the program says so and ends with return 1.

`in >> city >> temp` reads a word and a number, just like std::cin. It also returns the stream itself, which counts as false once nothing more can be read, so while (in >> city >> temp) goes through the file to the end. Along the way the cities are counted and the hottest one is found.

To read whole lines, spaces included, use std::getline(in, line) in a while, as with the keyboard in lesson 5. When you run the example in DMarlon Studio, temperatures.txt appears next to the program.

Code · main.cpp

#include <fstream>#include <iostream>#include <string>int main() {    // ofstream writes a file in the current folder (it creates or empties it).    std::ofstream out("temperatures.txt");    out << "Madrid 24.5\n" << "Bilbao 17.5\n" << "Sevilla 31.2\n";    out.close();    // std::ios::app adds at the end instead of starting again.    std::ofstream more("temperatures.txt", std::ios::app);    more << "Valencia 26.8\n";    more.close();    // ifstream reads it; always check that it could be opened.    std::ifstream in("temperatures.txt");    if (!in) { std::cout << "Could not open the file\n"; return 1; }    // >> reads word by word, just like std::cin, until the end of the file.    std::string city, hottestCity;    double temp, hottest = -100;    int count = 0;    while (in >> city >> temp) {        std::cout << city << ": " << temp << " C\n";        if (temp > hottest) { hottest = temp; hottestCity = city; }        count++;    }    std::cout << count << " cities, the hottest is " << hottestCity << " (" << hottest << " C)\n";    return 0;}

Output · c++ -std=c++17 main.cpp -o main && ./main

Madrid: 24.5 C
Bilbao: 17.5 C
Sevilla: 31.2 C
Valencia: 26.8 C
4 cities, the hottest is Sevilla (31.2 C)

Key points

std::ofstream f("name") creates or empties the file for writing.
std::ios::app appends without erasing what is there.
std::ifstream f("name") opens for reading; check if (!f).
Write with <<, read with >> or std::getline.
while (f >> x) reads until the end of the file.

Exercise

After the loop, open the file again with a new std::ifstream and use while (std::getline(again, line)) to print each line with a number: 1: Madrid 24.5, 2: Bilbao 17.5...