Read file line by line using ifstream in C++

Question

Read file line by line using ifstream in C++

The contents of file.txt are:

Where 5 3 is an organised pair. How would I deal with this information line by line in C++?
I'm ready to get the first line, however, how would I get the following line of the file?

ifstream myfile;

myfile.open ("file.txt");

ofstream file-io cpp

4 years ago by vishaljlf39

3 Answers

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**espadacoder11** · Answer 1 · 2021-05-17T10:14:19

First, make an ifstream:

#include <fstream>
std::ifstream infile("thefile.txt");
The two standard methods are:

1. Assume that every line consists of two numbers and read token by token*:

int a, b;
while (infile >> a >> b)
{
// process pair (a,b)
}

2. *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.

**kshitijrana14** · Answer 2 · 2021-05-18T11:33:10

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.

**RoliMishra** · Answer 3 · 2021-06-01T13:54:10

Since your coordinates belong together as pairs, why not write a struct for them?

struct CoordinatePair

{

    int x;

    int y;

};

Then you can write an overloaded extraction operator for istreams:

std::istream& operator>>(std::istream& is, CoordinatePair& coordinates)

{

    is >> coordinates.x >> coordinates.y;



    return is;

}

And then you can read a file of coordinates straight into a vector like this:

#include <fstream>

#include <iterator>

#include <vector>



int main()

{

    char filename[] = "coordinates.txt";

    std::vector<CoordinatePair> v;

    std::ifstream ifs(filename);

    if (ifs) {

        std::copy(std::istream_iterator<CoordinatePair>(ifs), 

                std::istream_iterator<CoordinatePair>(),

                std::back_inserter(v));

    }

    else {

        std::cerr << "Couldn't open " << filename << " for reading\n";

    }

    // Now you can work with the contents of v

}

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Read file line by line using ifstream in C++

3 Answers

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