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please try the code below

note : it finds all the files having "bmp" extension at the working directory and loads as gray-scale then saves as "original-image-name + pgm"

#include "opencv2/imgproc.hpp"
#include "opencv2/highgui.hpp"

using namespace cv;
using namespace std;

int main()
{
    vector<String> filenames;
    String folder = "*.bmp";

    glob(folder, filenames);

    for(size_t i = 0; i < filenames.size(); ++i)
    {
        Mat src = imread(filenames[i],0); // open image as gray

        if(src.data)
        {
            imwrite( filenames[i]+".pgm", src );
        }
    }
    return 0;
}