C ++将CSV文件的一列写入向量

Question

I have a CSV file with a bunch of columns, but I only need the information for the 11th column. How do I read through each line and skip to the 11th column in each line? I'm struggling to find clear information on how to read files in c++. This is what I have so far:

#include<iostream>
#include<fstream>
#include<vector>
#include <sstream>
#include <string>

std::string readStock(std::string fileName){
   std::vector<std::string> ticker; //create vector
   std::ifstream f(fileName, std::ios::in|std::ios:: binary|std::ios::ate);
   std::string finalString = "";
   if(f.is_open()){
     std::string str;
     std::getline(f,str); //skip the first row
     while(std::getline(f,str)){ //read each line
       std::istringstream s(str);  //stringstream to parse csv
       std::string val;  //string to hold value
       for(int i=1;i<=10;++i){ //skips everything until we get to the
column that we want
         while(std::getline(s,val, ',')){
         }
       std::getline(s,val,',');
       ticker.push_back(val);
     }
     f.close();

     finalString = ticker.front();
   }
}
   else{
     finalString="Could not open the file properly.";
   }
   return finalString;
}
int main(){
   std::string st;
   st=readStock("pr.csv");
   std::cout<<st<<std::endl;
   return 0;
}

Answer 1

There is a very simple solution for your problem.

You define a proxy class that reads one complete line, splits it into ALL tokens, using the dedicated functionality of the std::regex_token_iterator and then extracts the 11th element.

Using this proxy mechanism, you can use the std::istream_iterator to read the complete file, column 11, into a std::vector . For that we use the range constructor of the std::vector .

The result is a simple and short one-liner.

Please see:

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

std::regex delimiter{ "," };
constexpr size_t targetColumn = 10U; // Target column is eleven


struct String11 {   // Proxy for the input Iterator

    // Overload extractor. Read a complete line
    friend std::istream& operator>>(std::istream& is, String11& s11) { 

        // Read a complete line
        if (std::string line{}; std::getline(is, line)) {

            // Split it into tokens
            std::vector token(std::sregex_token_iterator(line.begin(), line.end(), delimiter, -1), {});
            // We only need one column
            if (targetColumn < token.size()) {
                // Get column 11
                s11.result = token[targetColumn];
            }
        }
        return is; 
    }

    // Cast the type 'String11' to std::string
    operator std::string() const { return result; }

    // Temporary to hold the resulting string
    std::string result{};
};

int main() {

    // Open CSV fíle
    if (std::ifstream csvFile{ "pr.csv" }; csvFile) {

        // Read complete CSV file and get column 11 of each line
        std::vector col11(std::istream_iterator<String11>(csvFile), {});

        // Show output. Show all columns 11
        std::copy(col11.begin(), col11.end(), std::ostream_iterator<std::string>(std::cout, "\n"));
    }
    return 0;
}

EDIT:

For having output with doubles.

We just change one line in the cast operator in the proxy. That's all.

Even in main, there is no change in the read operatrion necessary. Through CTAD, the vector will be of type double .

Please see:

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

std::regex delimiter{ "," };
constexpr size_t targetColumn = 10U; // Target column is eleven


struct String11 {   // Proxy for the input Iterator

    // Overload extractor. Read a complete line
    friend std::istream& operator>>(std::istream& is, String11& s11) { 

        // Read a complete line
        if (std::string line{}; std::getline(is, line)) {

            // Split it into tokens
            std::vector token(std::sregex_token_iterator(line.begin(), line.end(), delimiter, -1), {});
            // We only need one column
            if (targetColumn < token.size()) {
                // Get column 11
                s11.result = token[targetColumn];
            }
        }
        return is; 
    }

    // Cast the type 'String11' to double
    operator double() const { return std::stod(result); }

    // Temporary to hold the resulting string
    std::string result{};
};

int main() {

    // Open CSV fíle
    if (std::ifstream csvFile{ "r:\\pr.csv" }; csvFile) {

        // Read complete CSV file and get column 11 of each line
        std::vector col11(std::istream_iterator<String11>(csvFile), {});

        // Show output. Show all columns 11
        std::copy(col11.begin(), col11.end(), std::ostream_iterator<double>(std::cout, "\n"));
    }
    return 0;
}

Output needs to adapted as well.