CODEWITHSUNDEEP
c++file-io
Chris
Chris

Reputation: 2060

Reading file in specific format

I have a rather simple C++-problem, but coming from a C-background I am not really aware of all the I/O capabilities of C++. So here is the problem:

I have a simple .txt file with a specific format, the textfile looks like this:

123 points are stored in this file
pointer number | x-coordinate | y-coordinate
0      1.123      3.456
1      2.345      4.566
.....

I want to read out the coordinates. How can I do this? The first step is fine with:

int lines;
ifstream file("input.txt");
file >> lines;

This stores the first number in the file (i.e. the 123 in the example) in lines. Now I'd like to iterate over the file and only read the x and y coordinates. How can I do this efficently?

Upvotes: 3

Views: 1716

Answers (3)

ildjarn
ildjarn

Reputation: 62975

#include <cstddef>
#include <limits>
#include <string>
#include <vector>
#include <fstream>

struct coord { double x, y; };

std::vector<coord> read_coords(std::string const& filename)
{
    std::ifstream file(filename.c_str());
    std::size_t line_count;
    file >> line_count;

    // skip first two lines
    file.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    file.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

    std::vector<coord> ret;
    ret.reserve(line_count);
    std::size_t pointer_num;
    coord c;
    while (file >> pointer_num >> c.x >> c.y)
        ret.push_back(c);
    return ret;
}

Add error handling where appropriate.

Upvotes: 3

Wicked Coder
Wicked Coder

Reputation: 1118

Use the while loop

char buffer[256];  

while (! file.eof() )  

   {  

     myfile.getline (buffer,100);  

     cout << buffer << endl;  

   }

and then you need to parse out your buffer.

EDIT: The correct for using a while loop with eof is

while ((ch = file.get()) != EOF) {

}

Upvotes: -1

Jerry Coffin
Jerry Coffin

Reputation: 490108

I'd probably do it just about like I would in C, just using iostreams:

std::ifstream file("input.txt");

std::string ignore;
int ignore2;
int lines;
double x, y;

file >> lines;
std::getline(ignore, file);   // ignore the rest of the first line
std::getline(ignore, file);   // ignore the second line

for (int i=0; i<lines; i++) {
     file >> ignore2 >> x >> y;    // read in data, ignoring the point number
     std::cout << "(" << x << "," << y << ")\n";   // show the coordinates.
}

Upvotes: 4

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