Line reading

Learn to read line by line with getline.

Reading lines with getline

std::getline reads from a file up to the end of a line, not to whitespace. This way you get lines that contain spaces in full.

Example: std::string line; std::getline(in, line);. This reads one line from the in stream and stores it in line. >> would stop at a space; getline stops at a newline and discards it. To process all lines in a file, while (std::getline(in, line)) is common: it loops once per successful read and stops at the end. This is the standard way to process text files line by line.

The getline loop

To process all lines in a file use while (std::getline(in, line)). The loop runs while reading succeeds and stops at the end of the file.

Example: while (std::getline(in, line)) { std::cout << line << "\n"; }. This prints every line in the file one below the other. The loop variable line is refilled on each iteration. Empty lines are also read by getline; in that case line is an empty string. Calling in.close(); after the loop to close the file is a good habit.

Exercise Read the first line and print it

The starting code writes two lines to data.txt. Open the file with std::ifstream, read the first line with std::getline and print it with std::cout.

Show example solution

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

int main() {
    std::ofstream out("data.txt");
    out << "First line\nSecond line";
    out.close();

    std::ifstream in("data.txt");
    std::string line;
    std::getline(in, line);
    std::cout << line;
    return 0;
}

Test yourself

What does std::getline(in, line) do?
```
std::string line;
std::getline(in, line);
```
- Reads up to the end of the line
- Reads one number
- Reads one character
- Closes the file
Show answer

Reads up to the end of the line — getline reads up to the end of the line.
Match each piece to its role

std::getline

Line reading
>>

Word reading
in.close()

Closing the file
while (getline(...))

Line loop

Complete the getline call

std::____(in, line);

Show answer

std::getline(in, line);

Next lesson: Summary - File I/O

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