Getline to read data from txt file

Question

I'm having a bit of a problem with extracting data from a simple .txt file with the getline command.

The txt file is very simple: a column of 400 numbers. I use a vector to store them with the following code:

int i = 0;
string line;
vector <double> vec;

while (getline(input, line))
{
    vec.push_back(i);
    N++;
    input >> vec[i];
    i++;
} 

It correctly creates a vector of 400 elements but first line of txt file is ignored (I end up with vec[0] = 2nd line of txt file instead of 1st) and 399th element is 399 instead of the 400th line of txt file.

I tried several other ways to extract this data but it was unsuccessful.

Thank you for your help!

EDIT:

I have edited the code according to some of the remarks:

vector <double> vec;
string line;
double num;

while (getline(input, line))
{
    input >> num;
    vec.push_back(num);
}

Unfortunately, it still skips the first line of my text file.

EDIT 2 --> SOLUTION:

Thanks to all of your remarks, I realized that I was doing something wrong when using both getline and input >> num;

Here is how the problem was solved:

double num;
vector <double> vec;

while (input >> num)
{
    vec.push_back(num);
}

Answer 1

You can read the entire file into a vector just by passing std::istream_iterator to std::vector constructor, without loops:

std::vector<int> v{
    std::istream_iterator<int>{input}, 
    std::istream_iterator<int>{}
};

Eg:

#include <iostream>
#include <iterator>
#include <vector>
#include <exception>

template<class T>
std::vector<T> parse_words_into_vector(std::istream& s) {
    std::vector<T> result{
        std::istream_iterator<T>{s},
        std::istream_iterator<T>{}
    };
    if(!s.eof())
        throw std::runtime_error("Failed to parse the entire file.");
    return result;
}

int main() {
    auto v = parse_words_into_vector<int>(std::cin);
    std::cout << v.size() << '\n';
}

Answer 2

You loose the first line due to reading from the file once more - here:

while (getline(input, line))
    // ^^^^^^^ Here you read the first line
{
    input >> num;
 // ^^^^^^^^ Here you will read the second line

You told you want a vector of doubles - like:

std::vector<double> vec;

So you should use std::stod to convert the line read by getline into a double. Like:

while (std::getline(input, line))
{
    // Convert the text line (i.e. string) to a floating point number (i.e. double)
    double tmp;
    try
    {
        tmp = stod(line);
    }
    catch(std::invalid_argument)
    {
        // Illegal input
        break;
    }
    catch(std::out_of_range)
    {
        // Illegal input
        break;
    }

    vec.push_back(tmp);
} 

Don't do input >> num; inside the loop.

If you really want to use input >> num; then you shall not use getline . That is - you can use either but not both.

Answer 3

Change your while loop like below:-

while (getline(input, line))
{
    vec.push_back(line);
    //N++;
    //input >> vec[i];
    //i++;
} 

Also try with below option

 do{
    vec.push_back(i);
    //N++;
    //i++;
 }while (input >> vec[i++]);

Answer 4

You are starting by putting 0 in vector in first iteration:

vec.push_back(i);

then after you read first line you read next string, but stream get pointer from file is already in different place, so you override this 0 and skip first value from stream. What is worse that is oddly converted to double:

input >> vec[i];

This way you will get wrong. Try this:

while (std::getline(file, line)) {
    //In c++11
    vec.emplace_back(std::stod(line));
    //In c++ 98, #include <stdlib.h> needed
    //vec.push_back(atof(line.c_str())); 
}

This assumes you will always have proper file.