C++ string parsing (python style)

Question

I love how in python I can do something like:

points = []
for line in open(\"data.txt\"):
    a,b,c = map(float, line.split(\',\'))
    points += [(a,b,c)]
<         


        

Answer 1

I`d do something like this:

ifstream f("data.txt");
string str;
while (getline(f, str)) {
    Point p;
    sscanf(str.c_str(), "%f, %f, %f\n", &p.x, &p.y, &p.z); 
    points.push_back(p);
}

x,y,z must be floats.

And include:

#include <iostream>
#include <fstream>

Answer 2

Fun with Boost.Tuples:

#include <boost/tuple/tuple_io.hpp>
#include <vector>
#include <fstream>
#include <iostream>
#include <algorithm>

int main() {
    using namespace boost::tuples;
    typedef boost::tuple<float,float,float> PointT;

    std::ifstream f("input.txt");
    f >> set_open(' ') >> set_close(' ') >> set_delimiter(',');

    std::vector<PointT> v;

    std::copy(std::istream_iterator<PointT>(f), std::istream_iterator<PointT>(),
             std::back_inserter(v)
    );

    std::copy(v.begin(), v.end(), 
              std::ostream_iterator<PointT>(std::cout)
    );
    return 0;
}

Note that this is not strictly equivalent to the Python code in your question because the tuples don't have to be on separate lines. For example, this:

1,2,3 4,5,6

will give the same output than:

1,2,3
4,5,6

It's up to you to decide if that's a bug or a feature :)

Answer 3

You could read the file from a std::iostream line by line, put each line into a std::string and then use boost::tokenizer to split it. It won't be quite as elegant/short as the python one but a lot easier than reading things in a character at a time...

Answer 4

This answer is based on the previous answer by j_random_hacker and makes use of Boost Spirit.

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <boost/spirit.hpp>

using namespace std;
using namespace boost;
using namespace boost::spirit;

struct Point {
    double a, b, c;
};

int main(int argc, char **argv) 
{
    vector<Point> points;

    ifstream f("data.txt");

    string str;
    Point p;
    rule<> point_p = 
           double_p[assign_a(p.a)] >> ',' 
        >> double_p[assign_a(p.b)] >> ',' 
        >> double_p[assign_a(p.c)] ; 

    while (getline(f, str)) 
    {
        parse( str, point_p, space_p );
        points.push_back(p);
    }

    // Do something with points...

    return 0;
}

Answer 5

The C++ String Toolkit Library (StrTk) has the following solution to your problem:

#include <string>
#include <deque>
#include "strtk.hpp"

struct point { double x,y,z; }

int main()
{
   std::deque<point> points;
   point p;
   strtk::for_each_line("data.txt",
                        [&points,&p](const std::string& str)
                        {
                           strtk::parse(str,",",p.x,p.y,p.z);
                           points.push_back(p);
                        });
   return 0;
}

More examples can be found Here

Answer 6

All these good examples aside, in C++ you would normally override the operator >> for your point type to achieve something like this:

point p;
while (file >> p)
    points.push_back(p);

or even:

copy(
    istream_iterator<point>(file),
    istream_iterator<point>(),
    back_inserter(points)
);

The relevant implementation of the operator could look very much like the code by j_random_hacker.