Ibnu Syuhada Ibnu Syuhada - 2 months ago 7
C++ Question

How To Parse String File Txt Into Array With C++

I am trying to write a C++ program, but I am not familiar with C++. I have a

.txt
file, which contains values as follows:

0
0.0146484
0.0292969
0.0439453
0.0585938
0.0732422
0.0878906


What I have done in my C++ code is as follows:

#include <iostream>
#include <fstream>

using namespace std;

int main()
{
string line;
ifstream myReadFile;
myReadFile.open("Qi.txt");
if(myReadFile.is_open())
{
while(myReadFile.good())
{
getline(myReadFile,line);
cout << line << endl;
}
myReadFile.close();
}
return 0;
}


I would like to make the output of the program an array, i.e.

line[0] = 0
line[1] = 0.0146484
line[2] = 0.0292969
line[3] = 0.0439453
line[4] = 0.0585938
line[5] = 0.0732422
line[6] = 0.0878906

Answer

Assuming you want your data stored as floating point numbers (not strings) you probably want to do something like this:

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

int main() { 
    std::ifstream in("Qi.txt");

    // initialize the vector from the values in the file:
    std::vector<double> lines{ std::istream_iterator<double>(in),
                               std::istream_iterator<double>() };

    // Display the values:
    for (int i=0; i<lines.size(); i++)
         std::cout << "lines[" << i << "] = " << lines[i] << '\n';
}

Just a quick note on style: I prefer to see variables fully initialized right when you create them, so std::ifstream in("Qi.txt"); is preferable to std::ifstream in; in.open("Qi.txt");. Likewise, it's preferable to initialize the vector of lines directly from istream iterators rather than create an empty vector, then fill it in an explicit loop.

Finally, note that if you insist on writing an explicit loop anyway, you never want to use something like while (somestream.good()) or while (!somestream.eof()) to control your loop -- these are mostly broken, so they don't (dependably) read a file correctly. Depending on the type of data involved, they'll frequently appear to read the last item from the file twice. Usually, you want something like while (file >> value) or while (std::getline(file, somestring)). These check the state of the file immediately after reading, so as soon as reading fails they fall out of the loop, avoiding the problems of the while (good()) style.

Oh, as a side note: this is written expecting a compiler that (at lest sort of) conforms with C++11. For an older compiler you'd want to change this:

    // initialize the vector from the values in the file:
    std::vector<double> lines{ std::istream_iterator<double>(in),
                               std::istream_iterator<double>() };

...to something like this:

    // initialize the vector from the values in the file:
    std::vector<double> lines(( std::istream_iterator<double>(in)),
                                std::istream_iterator<double>() );