Reading data from a file in C++

Question

I'm trying to read data from a file line by line and push them back to a vector The data is of the format 123 23 45 19 46 22 87 26 31

The program gives no output and an error code of 11.

std::vector<int>* readDataFromFile(std::string path){


    std::vector<int>* v = new std::vector<int>;
    int a, b, c;
    std::ifstream inputStream;
    inputStream.open(path);
    while(inputStream>>a>>b){
        v->push_back(a);
        v->push_back(b);
        v->push_back(c);
    }
    inputStream.close();

}

int main() {

    std::vector<int>* v = readDataFromFile("file1.txt");
    for(auto it= v->begin(); it != v->end(); it++){
        std::cout<<*it<<std::endl;
    }

    return 0;
}

Are you sure the file is in the same directory the program is running? — πάντα ῥεῖ, Oct 27 '15 at 14:07
You should check if the `inputStream.is_open()`. Also you never take in `c` in `while(inputStream>>a>>b)` — NathanOliver, Oct 27 '15 at 14:08
That was gross, the return statement and the c did it. Apologies. — Antithesis, Oct 27 '15 at 14:11

Simón González Guerra · Accepted Answer · 2015-10-27T14:41:19.517

Don't forget to read c in the loop:

while(inputStream>>a>>b>>c) {
    v->push_back(a);
    v->push_back(b);
    v->push_back(c);
}

You are allocating a std::vector dynamically and never deleting it. Instead return the vector by value.
And also the call to fstream::close is not necessary, the stream is closed from fstream's destructor when it goes out of scope.

In general it is better to pass read-only string parameters by const reference.

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

std::vector<int> readDataFromFile(const std::string& path){
  std::vector<int> v;
  int a, b, c;
  std::ifstream inputStream(path);
  while(inputStream>>a>>b>>c){
      v.push_back(a);
      v.push_back(b);
      v.push_back(c);
  }

  return v;
}

int main() {
  std::vector<int> v = readDataFromFile("file1.txt");
  for(auto it = v.begin(); it != v.end(); it++){
      std::cout<<*it<<std::endl;
  }

  return 0;
}

It is worth noting that returning an `std::vector` by value may copy the entire vector, depending on the optimizations the compiler is able to perform. — Jørgen Fogh, Oct 27 '15 at 14:47
This is true, but it is a common practice to return vectors by value and rely on RVO (or in C++11 move semantics). See this question: https://stackoverflow.com/questions/11247654/returning-stdvector-by-value — Simón González Guerra, Oct 27 '15 at 14:58

Reading data from a file in C++

1 Answers1