C++ code for reading MNIST data-set

Here’s a code for reading MNIST dataset in C++, the dataset can be found HERE, and the file format is as well.

Using this code, you can read MNIST dataset into a double vector, or an OpenCV Mat, or Armadillo mat.

Feel free to use it for any purpose. (part of this code is stolen from HERE)

// readMNIST.cc// read MNIST data into double vector, OpenCV Mat, or Armadillo mat// free to use this code for any purpose// author : Eric Yuan // my blog: http://eric-yuan.me/// part of this code is stolen from http://compvisionlab.wordpress.com/#include "opencv2/core/core.hpp"#include "opencv2/imgproc/imgproc.hpp"#include "opencv2/highgui/highgui.hpp"#include <armadillo>#include <math.h>#include <iostream>using namespace cv;using namespace std;using namespace arma;int ReverseInt (int i){    unsigned char ch1, ch2, ch3, ch4;    ch1 = i & 255;    ch2 = (i >> 8) & 255;    ch3 = (i >> 16) & 255;    ch4 = (i >> 24) & 255;    return((int) ch1 << 24) + ((int)ch2 << 16) + ((int)ch3 << 8) + ch4;}void read_Mnist(string filename, vector<vector<double> > &vec){    ifstream file (filename, ios::binary);    if (file.is_open())    {        int magic_number = 0;        int number_of_images = 0;        int n_rows = 0;        int n_cols = 0;        file.read((char*) &magic_number, sizeof(magic_number));        magic_number = ReverseInt(magic_number);        file.read((char*) &number_of_images,sizeof(number_of_images));        number_of_images = ReverseInt(number_of_images);        file.read((char*) &n_rows, sizeof(n_rows));        n_rows = ReverseInt(n_rows);        file.read((char*) &n_cols, sizeof(n_cols));        n_cols = ReverseInt(n_cols);        for(int i = 0; i < number_of_images; ++i)        {            vector<double> tp;            for(int r = 0; r < n_rows; ++r)            {                for(int c = 0; c < n_cols; ++c)                {                    unsigned char temp = 0;                    file.read((char*) &temp, sizeof(temp));                    tp.push_back((double)temp);                }            }            vec.push_back(tp);        }    }}void read_Mnist(string filename, vector<cv::Mat> &vec){    ifstream file (filename, ios::binary);    if (file.is_open())    {        int magic_number = 0;        int number_of_images = 0;        int n_rows = 0;        int n_cols = 0;        file.read((char*) &magic_number, sizeof(magic_number));        magic_number = ReverseInt(magic_number);        file.read((char*) &number_of_images,sizeof(number_of_images));        number_of_images = ReverseInt(number_of_images);        file.read((char*) &n_rows, sizeof(n_rows));        n_rows = ReverseInt(n_rows);        file.read((char*) &n_cols, sizeof(n_cols));        n_cols = ReverseInt(n_cols);        for(int i = 0; i < number_of_images; ++i)        {            cv::Mat tp = Mat::zeros(n_rows, n_cols, CV_8UC1);            for(int r = 0; r < n_rows; ++r)            {                for(int c = 0; c < n_cols; ++c)                {                    unsigned char temp = 0;                    file.read((char*) &temp, sizeof(temp));                    tp.at<uchar>(r, c) = (int) temp;                }            }            vec.push_back(tp);        }    }}void read_Mnist(string filename, vector<arma::mat> &vec){    ifstream file (filename, ios::binary);    if (file.is_open())    {        int magic_number = 0;        int number_of_images = 0;        int n_rows = 0;        int n_cols = 0;        file.read((char*) &magic_number, sizeof(magic_number));        magic_number = ReverseInt(magic_number);        file.read((char*) &number_of_images,sizeof(number_of_images));        number_of_images = ReverseInt(number_of_images);        file.read((char*) &n_rows, sizeof(n_rows));        n_rows = ReverseInt(n_rows);        file.read((char*) &n_cols, sizeof(n_cols));        n_cols = ReverseInt(n_cols);        for(int i = 0; i < number_of_images; ++i)        {            arma::mat tp(n_rows, n_cols);            for(int r = 0; r < n_rows; ++r)            {                for(int c = 0; c < n_cols; ++c)                {                    unsigned char temp = 0;                    file.read((char*) &temp, sizeof(temp));                    tp(r, c) = (double) temp;                }            }            vec.push_back(tp);        }    }}void read_Mnist_Label(string filename, vector<double> &vec){    ifstream file (filename, ios::binary);    if (file.is_open())    {        int magic_number = 0;        int number_of_images = 0;        int n_rows = 0;        int n_cols = 0;        file.read((char*) &magic_number, sizeof(magic_number));        magic_number = ReverseInt(magic_number);        file.read((char*) &number_of_images,sizeof(number_of_images));        number_of_images = ReverseInt(number_of_images);        for(int i = 0; i < number_of_images; ++i)        {            unsigned char temp = 0;            file.read((char*) &temp, sizeof(temp));            vec[i]= (double)temp;        }    }}void read_Mnist_Label(string filename, arma::colvec &vec){    ifstream file (filename, ios::binary);    if (file.is_open())    {        int magic_number = 0;        int number_of_images = 0;        int n_rows = 0;        int n_cols = 0;        file.read((char*) &magic_number, sizeof(magic_number));        magic_number = ReverseInt(magic_number);        file.read((char*) &number_of_images,sizeof(number_of_images));        number_of_images = ReverseInt(number_of_images);        for(int i = 0; i < number_of_images; ++i)        {            unsigned char temp = 0;            file.read((char*) &temp, sizeof(temp));            vec(i)= (double)temp;        }    }}int main(){//    string filename = "mnist/t10k-images-idx3-ubyte";//    int number_of_images = 10000;//    int image_size = 28 * 28;/*    //read MNIST iamge into Armadillo mat vector    vector<arma::mat> vec;    read_Mnist(filename, vec);    cout<<vec.size()<<endl;    cout<<vec[0].size()<<endl;    cout<<vec[0]<<endl;*//*    //read MNIST iamge into OpenCV Mat vector    vector<cv::Mat> vec;    read_Mnist(filename, vec);    cout<<vec.size()<<endl;    imshow("1st", vec[0]);    waitKey();*//*    //read MNIST iamge into double vector    vector<vector<double> > vec;    read_Mnist(filename, vec);    cout<<vec.size()<<endl;    cout<<vec[0].size()<<endl;*///    string filename = "mnist/t10k-labels-idx1-ubyte";//    int number_of_images = 10000;/*    //read MNIST label into double vector    vector<double> vec(number_of_images);    read_Mnist_Label(filename, vec);    cout<<vec.size()<<endl;*//*    //read MNIST label into armadillo colvec    //if you want rowvec, just use .t()    arma::colvec vec = arma::zeros<arma::colvec>(number_of_images);    read_Mnist_Label(filename, vec);    cout<<vec.size()<<endl;*/    return 0;}