if your data is from https://www.kaggle.com/c/facial-keypoints-detection, then rather try like this:

(avoid all string conversions in the 1st place)

#include <iostream>
#include <fstream>
using namespace std;

#include <opencv2/opencv.hpp>
using namespace cv;

int main(int argc, char **argv)
{
    const int imgsize = 96; // not 48, see: https://www.kaggle.com/c/facial-keypoints-detection/data?test.zip

    vector<int> ids;
    vector<Mat> imgs;

    ifstream in("test.csv");
    // skip 1st row:
    string dummy;
    getline(in, dummy);

    while(! in.bad())
for(int i=0; i<1783; i++)
    {
        // read id:
        int id;
        in >> id;
        ids.push_back(id);

        // skip the bloody comma ;(
        char comma;
        in >> comma;

        // read pixels into a flat column:
        Mat img;
        for (int i=0; i<imgsize*imgsize; i++)
        {
            int pixel;
            in >> pixel;
            img.push_back(uchar(pixel));
        }
        // reshape to quad again:
        img = img.reshape(1,imgsize);
        imgs.push_back(img);

        cerr << id << endl;
        imshow("face", img);
        waitKey(200);
waitKey(2);
    }
    return 0;
}