巧用ifstream判断文件是否存在

　　最近在写手写数字的识别软件，训练样例数量巨大而且数字个数不唯一，有可能在中途粘出一部分做测试样例。因此写下面的脚本来获取文件名，之后丢到训练函数里。

 #include <algorithm>
 #include <iostream>
 #include <iomanip>
 #include <cstring>
 #include <climits>
 #include <complex>
 #include <fstream>
 #include <cassert>
 #include <cstdio>
 #include <bitset>
 #include <vector>
 #include <deque>
 #include <queue>
 #include <stack>
 #include <ctime>
 #include <set>
 #include <map>
 #include <cmath>

 #include <opencv.hpp>
 #include <opencv2/opencv.hpp>
 #include <opencv2/ml/ml.hpp>
 #include <opencv2/core/core.hpp>
 #include <opencv_modules.hpp>
 #include <opencv2/highgui/highgui.hpp>
 #include <opencv2/imgproc/imgproc.hpp>

 #define cvQueryHistValue_1D( hist, idx0 ) \
     ((float)cvGetReal1D( (hist)->bins, (idx0)))

 using namespace std;
 using namespace cv;

 const string rootDir("./sample/");
 const string jpg(".jpg");
 const string dirNames[] = {
     rootDir + "0/", rootDir + "1/", rootDir + "2/",
     rootDir + "3/", rootDir + "4/", rootDir + "5/",
     rootDir + "6/", rootDir + "7/", rootDir + "8/",
     rootDir + "9/"
 };

 inline void i2s(string& str, int i, int len = ) {
     stringstream ss;
     ss << setw(len) << setfill('') << i;
     str = ss.str();
 }

 void train(string fileName) {

 }

 void getFile(string dirName, int num) {
     static int sum = ;
     string tmp;
     string fileName;
     string snum;
     stringstream ss;
     int cur = ;

     tmp.clear();
     ss << num; ss >> snum;
     ifstream fileRead;
     for (; ; cur++) {
         i2s(tmp, cur);
         fileName = dirNames[num] + snum + "_" + tmp + jpg;
         if (cur == ) cout << "start file name : " << fileName << endl;
         fileRead.open(fileName);
         if (!fileRead) {
             cout << "end file name : " << fileName << endl;
             break;
         }
         fileRead.close();
         train(fileName);
     }
     sum += cur - ;
     cout << "current number of samples : " << sum << endl << endl;
 }

 int main() {
     int i = ;
     for (int i = ; i != ; i++) {
         getFile(dirNames[i], i);
     }
     return ;
 }