Why is getline not splitting up the the stringstream more than once?

April 17, 2022

When I’m trying to read data from a file, it is not updating the value of tempString2.

Instead of grabbing and splitting the new line, it just holds the last value of the first data line.

The reason I believe that the second getline is the issue is that the data is being read in, and the loops are running the exact number of times that I need them to.

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

void PopulateWindLog(int dataIndex, std::string fileName, int const headerVecSize)
{
    std::string tempString;
    std::string tempString2;
    std::stringstream tempStream;

    // open data file
    std::ifstream dataFile(fileName);

    // skip the header line
    std::getline(dataFile, tempString);

    // while there is still data left in the file
  
    while(std::getline(dataFile, tempString))
    {
        tempStream << tempString;
        std::cout << tempString << std::endl;

        for(int i = 0; i < headerVecSize; ++i)
        {
            std::getline(tempStream, tempString2, ',');
            std::cout << tempString2 << std::endl;

            if(i == dataIndex)
            {
                std::cout << tempString2 << " INSIDE IF STATEMENT !!!! " << std::endl;
            }
            
        }
    }
}

This is the dataset that I’m using. For replication, I’m looking for column ‘S’ (index 10). headerVecSize is 18.

WAST,DP,Dta,Dts,EV,QFE,QFF,QNH,RF,RH,S,SR,ST1,ST2,ST3,ST4,Sx,T
31/03/2016 9:00,14.6,175,17,0,1013.4,1016.9,1017,0,68.2,6,512,22.7,24.1,25.5,26.1,8,20.74
31/03/2016 9:10,14.6,194,22,0.1,1013.4,1016.9,1017,0,67.2,5,565,22.7,24.1,25.5,26.1,8,20.97
31/03/2016 9:20,14.8,198,30,0.1,1013.4,1016.9,1017,0,68.2,5,574,22.7,24,25.5,26.1,8,20.92

This is the output that I’m getting.
Output of the built program

>Solution :

std::getline() extracts characters from input and appends them to str until one of the following occurs (checked in the order listed)

end-of-file condition on input, in which case, getline sets eofbit.
the next available input character is delim, as tested by Traits::eq(c, delim), in which case the delimiter character is extracted from input, but is not appended to str.
str.max_size() characters have been stored, in which case getline sets failbit and returns.

(from https://en.cppreference.com/w/cpp/string/basic_string/getline)

So after reading all elements from your first input line, tempstream.good() is false. No further input is read.

Proposed solution: Move definition of tempstream into your while loop. You will get a stream in good state for each line.