Reading csv into array

Question

I know the topic has been discussed a lot but I still don't really get it. Admittedly I am pretty new to C++ coding, so please go easy on me :)

Anyhow: I have this examplatory csv file:

    ,313,315
91.5919,1.44421,1.74019
91.592,1.44254,1.73816
91.5921,1.43859,1.73336
91.5922,1.43449,1.73109

I need an array with unknown dimensions a priori, since the csv will expand unpredictably -> dynamic array in both x and y direction.

This is what I managed to do so far:

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

using namespace std;

int main()
{
    vector<vector<double> > values;
    vector<double> valueline;
    ifstream fin("in.csv");
    string item;
    for (string line; getline(fin, line); )
    {
        istringstream in(line);

        while (getline(in, item, ','))
        {
            valueline.push_back(atof(item.c_str()));
        }

        values.push_back(valueline);
        valueline.clear();
    }
}

It appears to be doing the job, but if I try to output the array or parts of it, I get strange results. For example cout << values[0][3] << endl; yields 1.63042e-322. values[0][4] yields 91.5919. Also sizeof(values[0]) is 24.

Am I doing something I am not supposed to here?

Any help would be great! Thanks

Amith · Accepted Answer · 2014-01-13 17:04:50Z

2

Try using .at() operator to see whether you aren't reading beyond the end of the vector? values[0][3] is the fourth element of the first line, which doesn't exist in your input?

You probably want to do:

std::cout << values[3][0]; // Fourth line, first element;

or

std::cout << values.at(3).at(0); // Fourth line, first element;

Index operators on vectors do not do bounds checking. This is for performance reasons. You are reading out of bounds, and thus you get the strange numbers. at() throws a std::out_of_range exception.

edited Jan 13, 2014 at 17:04

Amith

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answered Jan 13, 2014 at 16:39

tillaert

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Comments

Chris Dodd · Accepted Answer · 2014-01-13 17:13:54Z

1

Using sizeof on a vector is mostly useless -- it tells you the size of the top-level vector data structure (generally two size_ts and a pointer) in bytes, which is not terribly useful. If you want the size of the vector, use the size method -- values[0].size() tells you how many values are on the first line.

answered Jan 13, 2014 at 17:13

Chris Dodd

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