segmentation fault 11 error while importing csv

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;

You should read how [vector](http://en.cppreference.com/w/cpp/container/vector) works again. You are writing to a 0x0 matrix, thus the segfault. — Baum mit Augen, Jun 09 '14 at 19:56
I suggest you use `content.at(i).at(j) = 2;` Once you do that, when your code throws an exception and you've discovered you don't have adequate space reserved for your "matrix", rethink your algorithm, and likely review how `std::vector<>::operator[]` works, which likely isn't how you think. `matrix` has no rows, therefore your code invokes UB. — WhozCraig, Jun 09 '14 at 19:56

R Sahu · Accepted Answer · 2014-06-09T21:46:01.057

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;
}

Great answer, but I'm confused as to how `myfile >> line` will "Read a line as a string", as it stops reading on first encounter of whitespace. I *think* you meant `if (std::getline(myfile, line))` but its hard to tell. — WhozCraig, Jun 09 '14 at 21:38
@WhozCraig, that's what I meant. Thanks for pointing out the error. Fixed the answer. — R Sahu, Jun 09 '14 at 21:46

score 0 · Answer 2 · answered Jun 09 '14 at 19:57

0

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.

answered Jun 09 '14 at 19:57

Mr_Hic-up

389
2
11

score -1 · Answer 3 · answered Jun 12 '14 at 04:31

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;
}

-1 http://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-considered-wrong — πάντα ῥεῖ, Jun 12 '14 at 17:48

segmentation fault 11 error while importing csv

3 Answers3