Reading from text files C++

I have a text file containing three columns of numbers; one column each for the x,y,z coordinates of a bunch of points. All numbers are between 0 and 1.

I have created the following structure:

typedef struct 
{
    double xcd, ycd, zcd;
} point; 

I want to create a size-N array of structures of type point. Then I want to scan the text file line by line and for the nth particle, I want to put in the three numbers on the nth line int开发者_JAVA技巧o the respective xcd, ycd and zcd positions.

Tell me if there is some efficeint way of going about this.

---

Simply do it like has been shown five million billion kajillion times before, using ifstream, vector and various other accouterments.

ifstream infile("myfile.txt");

// check if file opened successfully
if (!infile) {
    cerr << "failure, can't open file" << endl;
    cin.get();
    return EXIT_FAILURE;
}

// the container in which we will store all the points
vector<point> points;

// a temporary point to hold the three coords in while we read them all
point tmp;

// read in three doubles from infile into the three coords in tmp
while (infile >> tmp.xcd && infile >> tmp.ycd && infile >> tmp.zcd)
    // add a copy of tmp to points
    points.push_back(tmp);

This will read in three doubles and put them in a point then put a copy of that point in points. However, if the number of numbers in the file modulus 3 is not 0, it will stop and not add the incomplete point to points.

---

Use a std::fstream.

If you're sure that the file is correct:

struct Point { 
    double xcd, ycd, zcd;
};
// btw this is how you should declare a structure in C++,
// the way you shown is rather characteristic to C and only used there

Point tab[N];

void foo() {
    std::ifstream f("file.txt");
    for (int i=0; i<N; ++i) {
        f >> tab[i].xcd >> tab[i].ycd >> tab[i].zcd;
    }    
}

If you're not sure that the file will exist and contain exactly this number of particles, you should check for f.fail() after each a read attempt.

---

I prefer using the standard generic algorithms to writing my own loops:

#include <iostream>
#include <vector>
#include <iterator>
#include <algorithm>
#include <fstream>

typedef struct 
{
  double xcd, ycd, zcd;
} point;

std::istream& operator>>(std::istream&is, point& pt)
{
  return is >> pt.xcd >> pt.ycd >> pt.zcd;
}

int main(int ac, char **av) {
  std::ifstream f("file.txt");

  std::vector<point> v;
  std::copy(
    std::istream_iterator<point>(f),
    std::istream_iterator<point>(),
    std::back_inserter(v));
}

---

Another design is to overload the stream extraction operator in your point structure:

struct Point
{
    double x;
    double y;
    double z;

    friend istream& operator>>(istream& inp, Point& p);
}

istream& operator>>(istream& inp, Point& p)
{
    inp >> x;
    inp >> y;
    inp >> z;
    inp.ignore(100000, '\n');
    return inp;
}

Usage:

ifstream myfile("data.txt");
Point p;
vector<Point> point_container;
while (myfile >> p)
{
    point_container.push_back(p);
}