reading data file into 2d array c++

Question

I have a text file with 2 columns and many rows. each column is separated by spaces. i need to read them to a 2D array for further calculations. my data file looks like

0.5 0.479425539
1   0.841470985
1.5 0.997494987
2   0.909297427
2.5 0.598472144
3   0.141120008
3.5 -0.350783228
4   -0.756802495
4.5 -0.977530118
5   -0.958924275  

And my feeble attempt is

#include <iostream>
#include <fstream>
#include <string>
#include <conio.h>
#include <ctype.h>
using namespace std;

int main () {
  char line,element;
  std::ifstream myfile ("C:\\Users\\g\\Desktop\\test.txt");
  if (myfile.is_open())
  {
    while ( myfile.good() )
    {
      getline(myfile,line);
       cout << line<<endl;               
      _getch();
    }
    myfile.close();

  }

  else cout << "Unable to open file"; 

  return 0;

}

The problem is I'm not able to read them correctly.... its either reading the whole line... if I specify the delimiter as 'space' then, its not reading the next row.

Pls point out whats wrong. and what should i do to store the data into 2d array for further calculations. Thank you

Answer 1

You can read the whole line into a std::string, then use std::istringstream to extract the values from the line.

---

A complete working program:

#include <iostream>
#include <string>
#include <sstream>
#include <fstream>

int main()
{
    std::ifstream file("C:\\Users\\g\\Desktop\\test.txt");

    std::string line;

    // Read a line of input from the file
    while (std::getline(file, line))
    {
        // `istringstream` behaves like a normal input stream
        // but can be initialized from a string
        std::istringstream iss(line);

        float value;

        // The input operator `>>` returns the stream
        // And streams can be used as a boolean value
        // A stream is "true" as long as everything is okay
        while (iss >> value)
        {
            std::cout << "Value = " << value << '\t';
        }

        // Flush the standard output stream and print a newline
        std::cout << std::endl;
    }
}

Given the contents in the file being as in the question, the first three lines of output should be:

Value = 0.5 Value = 0.479425539
Value = 1   Value = 0.841470985
Value = 1.5 Value = 0.997494987

---

For a 2d-array, I would use a std::vector of std::array:

#include <vector>
#include <array>

...

std::vector<std::array<float, 2>> array;

...

float value1, value2;
if (iss >> value1 >> value2)
{
    std::cout << "Values = " << value1 << ", " << value2;

    array.emplace_back(std::array<int, 2>{{value1, value2}});
}

Now the first line values are array[0][0] and array[0][1], and the last lines values are array[array.size() - 1][0] and array[array.size() - 1][1].

Answer 2

#include <fstream>
#include <string>
#include <sstream>
#include <iostream>
#include <vector>

int main(int argc, char** argv) {
   std::ifstream f(argv[1]);
   std::string l;
   std::vector<std::vector<double> > rows;
   while(std::getline(f, l)) {
       std::stringstream s(l);
       double d1;
       double d2;
       if(s >> d1 >> d2) {
           std::vector<double> row;
            row.push_back(d1);
            row.push_back(d2);
            rows.push_back(row);
        }
    }

    for(int i = 0; i < rows.size(); ++i)
        std::cout << rows[i][0] << " " << rows[i][1] << '\n';
}

The last for loop shows how to use the values in the "array". The variable rows is strictly speaking not an array, but a vector of vectors. However, a vector is much safer than c-style arrays, and allows access to its elements using [].

[As I posted this I saw a very similar program posted as a response. I wrote mine independently.]

Answer 3

As C++ has evolved over the years, below is a Modern C++ version.

• It uses auto where possible
• Uses std::pair to hold 2 values (A std::pair is a specific case of a std::tuple with two elements)
• Does not close file (destructor does that at end of block)
• Does not read line by line, as the stream uses <space> and <enter> as delimiters
• The variables have meaningful names, so the program "reads" easily,
• Uses a range for loop to output the data.
• Doesn't bring the whole std namespace into the code - Why is “using namespace std” considered bad practice?

.

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

int main( int argc, char** argv )
{
    if ( argc < 1 )
        return -1;

    const auto    fileName = argv[ 1 ];
    std::ifstream fileToRead( fileName );

    typedef std::pair< double, double > DoublesPair;
    std::vector< DoublesPair > rowsOfDoublesPair;
    DoublesPair                doublePairFromFile;

    while ( fileToRead >> doublePairFromFile.first >> doublePairFromFile.second )
    {
        rowsOfDoublesPair.push_back( doublePairFromFile );
    }

    for ( const auto row : rowsOfDoublesPair )
        std::cout << row.first << " " << row.second << '\n';
}