Read text file to build a vector.

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Hi, I am trying to read a text file "filename.txt" to build a vector of doubles. I used following code which works fine when I am using outside ROS. However, when I include this code as a part of my .cpp file in a ROS package, it does not work. At line while(myFile >> value), it does not read any thing and hence does not enter the loop. Any help is appreciated.

#include <fstream>
#include <vector>
#include <iostream>

std::vector<double> vec;

void readDATA(){
double value;
std::ifstream myFile;
myFile.open("filename.txt", std::ios::app); 
if (myFile.is_open()){
    std::cout << "File is open."<<std::endl;
    while(myFile >> value){
        vec.push_back(value);
        std::cout << "value is " <<value<< std::endl;
        }
    myFile.close();
}
else std::cout << "Unable to open the file"; 
}

int main(){
     readDATA();
return 0;
}