Read File Line by Line Using Ifstream in C++

Question

The contents of file.txt are:

Where 5 3 is a coordinate pair. How do I process this data line by line in C++?

I am able to get the first line, but how do I get the next line of the file?

ifstream myfile;
myfile.open ("file.txt");

Accepted Answer

First, make an ifstream:

#include <fstream>
std::ifstream infile("thefile.txt");

The two standard methods are:

Assume that every line consists of two numbers and read token by token:
```
int a, b;
while (infile >> a >> b)
{
    // process pair (a,b)
}
```

Line-based parsing, using string streams:

#include <sstream>
#include <string>

std::string line;
while (std::getline(infile, line))
{
    std::istringstream iss(line);
    int a, b;
    if (!(iss >> a >> b)) { break; } // error

    // process pair (a,b)
}

You shouldn't mix (1) and (2), since the token-based parsing doesn't gobble up newlines, so you may end up with spurious empty lines if you use getline() after token-based extraction got you to the end of a line already.

Answer 2

Use ifstream to read data from a file:

std::ifstream input( "filename.ext" );

If you really need to read line by line, then do this:

for( std::string line; getline( input, line ); )
{
    ...for each line in input...
}

But you probably just need to extract coordinate pairs:

int x, y;
input >> x >> y;

Update:

In your code you use ofstream myfile;, however the o in ofstream stands for output. If you want to read from the file (input) use ifstream. If you want to both read and write use fstream.

Answer 3

Reading a file line by line in C++ can be done in some different ways.

Must Read

[Fast] Loop with std::getline()

The simplest approach is to open an std::ifstream and loop using std::getline() calls. The code is clean and easy to understand.

#include <fstream>

std::ifstream file(FILENAME);
if (file.is_open()) {
    std::string line;
    while (std::getline(file, line)) {
        // using printf() in all tests for consistency
        printf("%s", line.c_str());
    }
    file.close();
}

Read File Line by Line Using Ifstream in C++

8 Answers

Must Read

[Fast] Loop with std::getline()

Alexander Ross