How to read in values from specific columns in a CSV file to perform calculations - C++

Question

so I'm trying to read in some data from a csv file into my program, but I only want some specific columns. I know there is a lot of stuff online regarding this but I can't seem to fit any of them to my requirements.

Firstly, I have created a struct that takes in Date and Time (class objects) and a Speed value from a file, and store it in Vector template class I have made myself. My main program right now works correctly for a test file like this:

31/12/2013 11:45,55.6

Date and time is read together, and then separated, and it works fine. Now here is the actual file:

WAST,DP,Dta,Dts,EV,QFE,QFF,QNH,RF,RH,S,SR,ST1,ST2,ST3,ST4,Sx,T
31/03/2016 09:00,14.6,175,17,0,1013.4,1016.9,1017,0,68.2,6,512,22.7,24.1,25.5,26.1,8,20.74
31/03/2016 09:10,14.6,194,22,0.1,1013.4,1016.9,1017,0,67.2,5,565,22.7,24.1,25.5,26.1,8,20.97
31/03/2016 09:20,14.8,198,30,0.1,1013.4,1016.9,1017,0,68.2,5,574,22.7,24,25.5,26.1,8,20.92

What I want to do is extract details from the WAST column which is date and time, and then the speeds from the S column. I have a generic idea of how to do it i.e: Extract the whole first line into a string and ignore everything other than WAST and S, or something like that. But even then, how am I supposed to get every value under only the columns I want to? Please help.

Main.cpp:

#include <iostream>
#include <string>
#include <fstream>
#include "Date.h"
#include "Time.h"
#include "Vector.h"

using namespace std;

typedef struct {

        Date d;
        Time t;
        float speed;

}WindLogType;

int main()
{

Date dTest;
Time tTest;
float speedtest = 52.5;

Vector<WindLogType> windlog;


ifstream infile("testinput2.csv");

if(!infile){

    cout << "File not found.";

    return -1;

};

WindLogType windlog2;

//int i = 0;

while(!infile.eof()){

    infile >> windlog2.d >> windlog2.t >> windlog2.speed;


    windlog.add(windlog2);

}

for(int i = 0; i < windlog.size(); i++){

    cout << windlog[i].d << " " << windlog[i].t << " Speed: " << windlog[i].speed << endl;

}


infile.close();

return 0;

}

Answer 1

What you can do here is write a stream extraction operator for your struct WindLogType that you use to read the file one line at a time, extracting the information you need from the line and populating the struct.

std::getline can be used to not only read a whole line, but also tokenise your string using a delimiter.

Here's something you can try with some comments to explain:

#include <fstream>
#include <string>
#include <sstream>

using Date = std::string;
using Time = std::string;

typedef struct {
    Date d;
    Time t;
    float speed;
} WindLogType;

std::istream& operator >> (std::istream& is, WindLogType& sl) {

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

        auto iss = std::istringstream{line};

        // Read date from line (up to space)
        std::getline(iss, sl.d, ' ');
        // Read time (up to ',')
        std::getline(iss, sl.t, ',');

        std::string speed;
        // Read speed (up to next comma)
        std::getline(iss, speed, ',');
        sl.speed = std::stof(speed);
    }

    return is;
}

int main()
{
    std::ifstream infile("testinput2.csv");

    if (!infile) {
        std::cout << "File not found.";
        return -1;
    };

    Vector<WindLogType> windlog;
    std::string str;
    std::getline(infile, str); // skip the first line

    WindLogType windlog2;
    while (infile >> windlog2) {
        windlog.add(windlog2);
    }

    return 0;
}

The stream extraction operator (>>) reads a line from the file. With that line it creates a std::istringstream object from it and then extracts the relevant information using std::getline.

I don't know how your Date and Time types are defined. I have just aliased them as std::string. You will have to do your own processing of the strings yourself.

Hope this is useful.

---

Edit:

The reason why you can't do while(infile >> windlog2.d >> windlog2.t >> windlog2.speed) as it that >> reads up to a white space/newline. Your data is bit complex: it's comma separated and you have parse the date and time fields. The good thing with the approach above is that it allows you to stream data directly into a WindLog structure.

If you really don't want to have a stream extraction operator for your type you can use the same code. It will be like this:

std::string line;
std::getline(infile, line); // skip the first line

while (std::getline(infile, line)) {
    WindLogType sl;
    auto iss = std::istringstream{ line };

    // Read date from line (up to space)
    std::getline(iss, sl.d, ' ');
    ...
}

Personally I prefer having a stream extraction operator for a type as it encapsulates all the reading operation in one function, but if you want to do some other processing then you can take the second approach.