Problem with converting vector of string to vector of doubles - c++

Question

I am trying to write a program that reads some numerical values from a .csv file, stores them in a std::vector<std::string>, and then converts these values into doubles and stores them in a std::vector<double>.
I am trying to do the conversion using stringstreams, which has worked fine for me in the past.
I have managed to import the numerical values and store them in the std::vector<std::string>, however I am getting a weird problem when trying to do the conversions to double. Only the very first value is stored in the std::vector<double> with a lot of significant figures missing, and the other entries are just ignored and not stored in the std::vector<double> at all.
Here is my code so far:
#include <string>
#include <fstream>
#include <iostream>
#include <sstream>
#include <vector>
double extract_double(std::string str)
{
std::stringstream ss;
double grade;
//Pass all the course details to string stream
ss << str;
//Extract the double type part of the string stream which is the course grade
ss >> grade;
str = "";
ss.ignore();
return grade;
}
int main()
{
std::ifstream my_input_file;
std::string file_name;
my_input_file.open("scale_free_gamma_2_fitnesses.csv");
int number_of_data_in_file;
std::vector<std::string> fitnesses_string;
std::vector<double> fitnesses;
std::string string_temp;
while (my_input_file.good()) {
//Extract strings from file
std::getline(my_input_file, string_temp, ',');
fitnesses_string.push_back(string_temp);
}
for (auto fitnesses_it = fitnesses_string.begin();
fitnesses_it < fitnesses_string.end(); ++fitnesses_it){
fitnesses.push_back(extract_double(*fitnesses_it));
}
for (auto fitnesses_itt = fitnesses.begin();
fitnesses_itt < fitnesses.end(); ++fitnesses_itt){
std::cout << *fitnesses_itt << std::endl;
}
return 0;
}

Answer 1

You should be reading individual lines from the file first, and then splitting up each line on commas.
And there are easier ways to handle the rest of your code, too.
Try something more like this instead:
#include <string>
#include <fstream>
#include <iostream>
#include <sstream>
#include <vector>
int main()
{
std::ifstream my_input_file("scale_free_gamma_2_fitnesses.csv");
std::vector<std::string> fitnesses_string;
std::vector<double> fitnesses;
std::string line, string_temp;
while (std::getline(my_input_file, line)) {
//Extract strings from line
std:::istringstream iss(line);
while (std::getline(iss, string_temp, ','))
fitnesses_string.push_back(string_temp);
}
for (const auto &s : fitnesses_string){
fitnesses.push_back(std:stod(s));
}
for (auto value : fitnesses){
std::cout << value << std::endl;
}
return 0;
}
In which case, even simpler would be to just get rid of the std::vector<std::string> altogether:
#include <string>
#include <fstream>
#include <iostream>
#include <sstream>
#include <vector>
int main()
{
std::ifstream my_input_file("scale_free_gamma_2_fitnesses.csv");
std::vector<double> fitnesses;
std::string line, string_temp;
while (std::getline(my_input_file, line)) {
//Extract strings from line
std:::istringstream iss(line);
while (std::getline(iss, string_temp, ','))
fitnesses.push_back(std:stod(string_temp));
}
for (auto value : fitnesses){
std::cout << value << std::endl;
}
return 0;
}

Answer 2

A few things. First, there's a far better way to loop over your vectors.
for (const std::string &str: fitnesses_string) {
}
These are referred to as range-based for-loops, and I think you can see how much cleaner this syntax is. In modern C++, you rarely need to use iterators anymore. (I won't say never, but rarely.)
Next, your inner loop makes no sense at all. Instead, just do it quite simply:
for (const std::string &str: fitnesses_string) {
fitnesses.push_back(extract_double(str));
}
Now, let's talk about how to convert a string to double.
fitnesses.push_back(std::stod(str));
So you don't need your method. There's already a method waiting for you. It's defined in #include <string>.