MNIST数据集转为.jpg图片格式

从mnist官网下载下来的mnist手写数据集是二进制文件流格式的，不能直接查看，如果需要查看，需要将二进制文件转化为jpg格式，可以用各种编程语言实现，如MATLAB、Python、C++等，本文是使用C++在Ubuntu 16.04操作系统下实现的，过程中出现许多问题，现记录如下：
1、问题描述
编译时出现一下错误

no matching function for call to std::basic_ifstream<char>::(std::string&)

原因：根据提示可以看出是读取文件ifstream时出现了问题，查找资料后知道是由于不同版本c++表达不同导致的，所以用g++编译器编译时要显式制定使用那个版本c++，本程序使用c++11，编译指令如下：

g++ -o b.out -std=c++11 mnist_bin2jpg2.cpp

2、问题：
编译时出现：

对‘cv::Mat::copySize(cv::Mat const&)’未定义的引用

原因：编译的时候没有链接到库文件，这样导致函数没有定义，所以变异的时候要手动链接到库文件

g++ -o b.out -std=c++11 mnist_bin2jpg2.cpp `pkg-config opencv --cflags --libs`

3、问题：
用ifstream读取文件时编译通过了，但是一直没有成功读取文件，一开始以为是因为文件夹包含有中文的原因，然后就用了只有英文名字的文件夹，仍就没有成功，找了很久，最后发现是自己文件名打错了（哭晕在厕所。。。）

最后附上网上找的再通过修改成功将mnist转为JPG图片的代码

    #include <iostream>      #include <fstream>      #include "opencv2/core/core.hpp"      #include "opencv2/highgui/highgui.hpp"      #include "opencv2/imgproc/imgproc.hpp"      #include <cstdio>    using namespace std;      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<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 = cv::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_Label(string filename, vector<int> &vec)      {    ifstream file(filename, ios::in | 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]= (int)temp;      }      }      }      string GetImageName(int number, int arr[])      {      string str1, str2;      for (int i = 0; i < 10; i++) {      if (number == i) {      arr[i]++;      char ch1[10];       sprintf(ch1, "%d", arr[i]);       str1 = std::string(ch1);      if (arr[i] < 10) {      str1 = "0000" + str1;      } else if (arr[i] < 100) {      str1 = "000" + str1;      } else if (arr[i] < 1000) {      str1 = "00" + str1;      } else if (arr[i] < 10000) {      str1 = "0" + str1;      }      break;      }      }      char ch2[10];      sprintf(ch2, "%d", number);      str2 = std::string(ch2);      str2 = str2 + "_" + str1;      return str2;      }      int main()      {      //reference: http://eric-yuan.me/cpp-read-mnist/      //test images and test labels      //read MNIST image into OpenCV Mat vector      string filename_test_images = "/home/linqingxiang/test/t10k-images-idx3-ubyte";      int number_of_test_images = 10000;      vector<cv::Mat> vec_test_images;      read_Mnist(filename_test_images, vec_test_images);      //read MNIST label into int vector      string filename_test_labels = "/home/linqingxiang/test/t10k-labels-idx1-ubyte";     vector<int> vec_test_labels(number_of_test_images);      read_Mnist_Label(filename_test_labels, vec_test_labels);      if (vec_test_images.size() != vec_test_labels.size()) {      cout<<"parse MNIST test file error"<<endl;      return -1;      }      //save test images      int count_digits[10];      for (int i = 0; i < 10; i++)      count_digits[i] = 0;      string save_test_images_path = "/home/linqingxiang/test/";      for (unsigned int i = 0; i <  vec_test_images.size(); i++) {      int number = vec_test_labels[i];      string image_name = GetImageName(number, count_digits);      image_name = save_test_images_path + image_name + ".jpg";      cv::imwrite(image_name, vec_test_images[i]);      }      //train images and train labels      //read MNIST image into OpenCV Mat vector      string filename_train_images = "/home/linqingxiang/test/train-images-idx3-ubyte";      int number_of_train_images = 60000;      vector<cv::Mat> vec_train_images;      read_Mnist(filename_train_images, vec_train_images);      //read MNIST label into int vector      string filename_train_labels = "/home/linqingxiang/test/train-labels-idx1-ubyte";      vector<int> vec_train_labels(number_of_train_images);      read_Mnist_Label(filename_train_labels, vec_train_labels);      if (vec_train_images.size() != vec_train_labels.size()) {      cout<<"parse MNIST train file error"<<endl;      return -1;      }      //save train images      for (int i = 0; i < 10; i++)      count_digits[i] = 0;      string save_train_images_path = "/home/linqingxiang/test/";      for (size_t i = 0; i < vec_train_images.size(); i++) {      int number = vec_train_labels[i];      string image_name = GetImageName(number, count_digits);      image_name = save_train_images_path + image_name + ".jpg";      cv::imwrite(image_name, vec_train_images[i]);      }      return 0;      }