nikjohn nikjohn - 18 days ago 5
C++ Question

Function reading from a file into vector C++

I have written a function that reads an unknown number of data (when in a column) from a file to a vector.

#include <iostream>
#include <vector>
#include <fstream> // file writing
#include <cassert>


void ReadFromFile(std::vector<double> &x, const std::string &file_name)
{
std::ifstream read_file(file_name);
assert(read_file.is_open());

size_t lineCount = 0;
while (!read_file.eof())
{
double temp;
read_file >> temp;
x.at(lineCount) = temp;
if (lineCount == x.size() - 1) { break; } // fixes the out of range exception

lineCount++;
}
read_file.close();
}
int main()
{
size_t Nx = 7;
size_t Ny = 7;
size_t Nz = 7;
size_t N = Nx*Ny*Nz;

// Initial Arrays
std::vector <double> rx(N);
std::string Loadrx = "Loadrx.txt";
ReadFromFile(rx, Loadrx);
}


But the lineCount is incrementing one extra time after the data from the file have been copied into the vector. Is there a more elegant way of fixing that problem than the if statement that I have written?

EDIT: Obviously, I will not upload the data file. The code compiles perfectly I am just looking for an improvement in the if statement (if one exists).

Answer

I have written a function that reads an unknown number of data (when in a column) from a file to a vector.

One of the most elegant (and, I suppose, idiomatic) ways to read unknown amount of data from a "column" (or otherwise regularly-formatted) file is to use istream iterators:

void ReadFromFile(std::vector<double> &x, const std::string &file_name)
{
    std::ifstream read_file(file_name);
    assert(read_file.is_open());

    std::copy(std::istream_iterator<double>(read_file), std::istream_iterator<double>(),
        std::back_inserter(x));

    read_file.close();
}

Usage:

int main()
{
    // Note the default constructor - we are constructing an initially empty vector.
    std::vector<double> rx;
    ReadFromFile(rx, "Loadrx.txt");
}

If you want to write a "safe" version with a limited number of elements to read, use copy_if:

void ReadFromFile(std::vector<double> &x, const std::string &file_name, unsigned int max_read)
{
    std::ifstream read_file(file_name);
    assert(read_file.is_open());

    unsigned int cur = 0;
    std::copy_if(std::istream_iterator<double>(read_file), std::istream_iterator<double>(),
    std::back_inserter(x), [&](const double&) {
        return (cur++ < max_read);
    });

    read_file.close();
}

Usage is obvious:

ReadFromFile(rx, Loadrx, max_numbers);