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[11] = {
    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 = 5) {
    stringstream ss;
    ss << setw(len) << setfill('0') << i;
    str = ss.str();
}

void train(string fileName) {

}

void getFile(string dirName, int num) {
    static int sum = 0;
    string tmp;
    string fileName;
    string snum;
    stringstream ss;
    int cur = 1;

    tmp.clear();
    ss << num; ss >> snum;
    ifstream fileRead;
    for (; ; cur++) {
        i2s(tmp, cur);
        fileName = dirNames[num] + snum + "_" + tmp + jpg;
        if (cur == 1) cout << "start file name : " << fileName << endl;
        fileRead.open(fileName);
        if (!fileRead) {
            cout << "end file name : " << fileName << endl;
            break;
        }
        fileRead.close();
        train(fileName);
    }
    sum += cur - 1;
    cout << "current number of samples : " << sum << endl << endl;
}


int main() {
    int i = 0;
    for (int i = 0; i != 10; i++) {
        getFile(dirNames[i], i);
    }
    return 0;
}