Alternative for deleted copy constrctor of ifstream - c++

Question

I want to extract data from a csv file but I have to get the number of rows and columns of the table first.
What I have so far is the following:
std::ifstream myfile(filename); // filename is a string and passed in by the constructor
if (myfile.is_open())
{
// First step: Get number of rows and columns of the matrix to initialize it.
// We have to close and re-open the file each time we want to work with it.
int rows = getRows(myfile);
std::ifstream myfile1(filename);
int columns = getColumns(myfile1);
if (rows == columns) // Matrix has to be quadratic.
{
std::ifstream myfile2(filename);
abwicklungsdreieck.set_Matrix(QuantLib::Matrix(rows, columns, 0)); // abwicklungsdreieck is initialised before
//...
}
else
{
std::cout << "\nNumber of rows has to equal number of columns.";
}
}
// [...]
int getRows(std::ifstream &myfile)
{
std::string line;
int rows = 0;
while (std::getline(myfile, line)) // While-loop simply counts rows.
{
rows++;
}
myfile.close();
return rows - 1;
}
int getColumns(std::ifstream &myfile)
{
std::string line;
char delimiter = ';';
size_t pos = 0;
int columns = 0;
while (std::getline(myfile, line) && columns == 0) // Consider first line in the .csv file.
{
line = line + ";";
while ((pos = line.find(delimiter)) != std::string::npos) // Counts columns.
{
line.erase(0, pos + 1);
columns++;
}
}
myfile.close();
return columns - 1;
}
This code is working. However, I have to open the file for three times which I do not like. Is there a way to evade this?
I was thinking about working with tempfiles in getRows() and getColumns() but the copying streams isn't possible since it doesn't make sense as I learned recently.
So, is there another way do that? Or can I for example evade the getline() and the line.erase() methods?

Answer 1

You can read the file line by line, convert each line to a stream, then read the columns on the stream:
std::ifstream myfile(filename);
if(!myfile) return 0;
std::string line;
while(std::getline(myfile, line))
{
std::stringstream ss(line);
std::string column;
while(std::getline(ss, column, ';'))
{
cout << column;
}
cout << "\n";
}
getline(myfile, line) will copy each row in to line.
Convert line to ss stream.
getline(ss, column, ';') will break the line in to columns.
Use std::stoi to convert the string in to integer.
If your matrix is based on std::vector, you can grow the vector one row at a time, so you don't need to know the size in advance.
#include <iostream>
#include <fstream>
#include <string>
#include <sstream>
#include <vector>
void readfile(const std::string &filename)
{
std::vector<std::vector<int>> matrix;
std::ifstream myfile(filename);
if(!myfile) return;
std::string buf;
while(std::getline(myfile, buf))
{
int maxrow = matrix.size();
std::stringstream ss(buf);
matrix.resize(maxrow + 1);
cout << "break in to columns:" << buf << "\n";
while(std::getline(ss, buf, ';'))
{
try {
int num = std::stoi(buf);
matrix[maxrow].push_back(num);
}
catch(...) { }
}
}
for(auto &row : matrix) {
for(auto col : row)
cout << col << "|";
cout << "\n";
}
}