segmentation fault 11 error while importing csv - c++

Question

I'm making a function importcsv() which takes in a filename and outputs a 2D array. For some reason, whenever I use the following version of importcsv(), the compiler runs smoothly, but the executable always returns a "segmentation fault: 11" error.
typedef vector<vector<double> > matrix;
matrix importcsv(string filename)
{
ifstream myfile (filename); //Constructs a stream, and then asssociates the stream with the file "filename"
matrix contents; // Vector which will store the contents of the stream.
int i, j;
while(!myfile.eof())
{
if(myfile.get()==','){++j;}
else if(myfile.get()=='\n'){++i; j=0;}
else{
contents[i][j]=2;}
}
return contents;
}
Can anyone find the source of the error? btw I have the following header:
#include <fstream>
#include <iostream>
#include <array>
#include <vector>
using namespace std;

Answer 1

You are getting "segmentation fault: 11" since you have not allocated memory for contents.
contents[i][j] will work only if contents has something in it.
You can divide reading of the file and constructing the matrix into various parts:
Reading all the numbers in a line and treating it as a row of contents.
Reading a number from the line and treating it as a column of a row.
This way, the program can be simplified. This also helps you easily isolate problems when there are any and fix them.
typedef vector<vector<double> > matrix;
double readNextNumber(std::istream& str)
{
double n = 0.0;
str >> n;
// Skip until we get ',' or '\n'
while (str)
{
int c = str.getc();
if ( c == ',' || c == '\n' || c == EOF )
break;
}
return n;
}
std::vector<double> importRow(std::ifstram& myfile)
{
std::string line;
std::vector<double> row;
// Read a line as a string.
// Then parse the string using std::istringstream.
// When you have finished parsing the line, you know
// you have finished constructing a row of the matrix.
std::getline(myfile, line);
if ( myfile )
{
std::istringstream str(line);
while (str)
{
double n = readNextNumber(str);
if (str)
{
row.push_back(n);
}
}
}
return row;
}
matrix importcsv(string filename)
{
ifstream myfile (filename); //Constructs a stream, and then asssociates the stream with the file "filename"
matrix contents; // Vector which will store the contents of the stream.
while(!myfile.eof())
{
std::vector<double> row = importRow(myfile);
if (myfile)
{
contents.push_back(row);
}
}
return contents;
}

Answer 2

You haven't defined the size of contents. So by default, it will be a vector of 0 element. Therefore the calls to the operator[] will lead to a segmentatin fault.

Answer 3

Implementing the advice of the others here, the quick fix is to use resize() before reading each value into the array:
//WARNING: THIS PROGRAM UTILIZES C++11
#include <fstream>
#include <iostream>
#include <array>
#include <vector>
#include <cctype>
#include <thread>
using namespace std;
typedef vector<vector<double> > matrix;
matrix importcsv(string filename)
{
ifstream myfile ("wavelengthtorgb.csv"); //Constructs a stream, and then asssociates the stream with the file "filename".
matrix contents {{0.0}};
char nextchar; double data; int i,j;
while(!myfile.eof())
{
myfile.get(nextchar);
if(nextchar==',')
{
++j;
contents[i].resize(j+1);
cout<<"encountered a comma."<<" contents is now " <<i+1<<" x "<<j+1<<'\n';
}
else if(isspace(nextchar))
{
myfile.get(); //You might not need this line - first run with this line, and if there is an error, delete it, and try again.
++i;
contents.resize(i+1);
j=0;
contents[i].resize(j+1);
cout<<"encountered a carriage return."<<" contents is now " <<i+1<<" x "<<j+1<<'\n';
}
else
{
myfile.unget();
myfile >> data;
contents[i][j]=data;
cout<< "encountered a double."<<" contents("<<i<<','<<j<<")="<<data<<'\n';
}
}
return contents;
}