C++ primer plus random.cpp

  1 // random.cpp -- random access to a binary file
  2 #include <iostream>     // not required by most systems
  3 #include <fstream>
  4 #include <iomanip>
  5 #include <cstdlib>      // (or stdlib.h) for exit()
  6 const int LIM = 20;
  7 struct planet
  8 {
  9     char name[LIM];      // name of planet
 10     double population;  // its population
 11     double g;           // its acceleration of gravity
 12 };
 13 
 14 const char * file = "planet.dat";  // ASSUMED TO EXIST (binary.cpp example)
 15 inline void eatline() { while (std::cin.get() != ‘\n‘) continue; }      //内联函数，编译的时候在调用的地方替换代码
 16                                                                         //用来解决函数调用的效率问题
 17 
 18 int main()
 19 {
 20     using namespace std;  //在主函数使用标准程序库的命名空间
 21     planet pl;
 22     cout << fixed;        //数字定点形式输出  
 23 
 24 // show initial contents
 25     fstream finout;         //建立文件流，名字叫做 finout 
 26     finout.open(file,
 27                 ios_base::in | ios_base::out | ios_base::binary);  /*调用finout子函数open，ios_base::in ,ios_base::out使用输入输出，i
 28                                                                         os_base::binary表示用二进制操作文件数据 */
 29     //NOTE: Some Unix systems require omitting | ios::binary
 30     int ct = 0;
 31     if (finout.is_open())    //finout.is_open()返回整数表示打开文件是否成功
 32     {
 33         finout.seekg(0);    // 将文件指针定位于文件的其实位置0（第一个字符）, FileNam.seekg(Num);
 34         cout << "Here are the current contents of the "
 35              << file << " file:\n";    
 36 
 37         /*finout.read读取文件，并都转化字符输出（原来为二进制） 
 38              FileName.read( (Type*) &struct,sizeof) */
 39         while (finout.read( (char *) &pl, sizeof pl))  
 40         {
 41             cout << ct++ << ": " << setw(LIM) << pl.name << ": "
 42                  << setprecision(0) << setw(12) << pl.population
 43                  << setprecision(2) << setw(6) << pl.g << endl;
 44         }
 45         if (finout.eof())  //到达文件尾清除错误信息
 46             finout.clear(); // clear eof flag
 47         else
 48         {
 49             cerr << "Error in reading " << file << ".\n";
 50             exit(EXIT_FAILURE);
 51         }
 52     }
 53     else
 54     {
 55         cerr << file << " could not be opened -- bye.\n";
 56         exit(EXIT_FAILURE);
 57     }
 58 
 59 // change a record
 60     cout << "Enter the record number you wish to change: ";
 61     long rec;
 62     cin >> rec;
 63     eatline();              // 吸收换行之前的输入  用于输入字符或者下次要输入字符的时候
 64     if (rec < 0 || rec >= ct)
 65     {
 66         cerr << "Invalid record number -- bye\n";
 67         exit(EXIT_FAILURE);
 68     }
 69     streampos place = rec * sizeof pl;  // 文件指针位置的数据类型  place = rec * sizeof pl 将place指向一个固定位置
 70     finout.seekg(place);    // random access
 71     if (finout.fail())
 72     {
 73         cerr << "Error on attempted seek\n";
 74         exit(EXIT_FAILURE);
 75     }
 76 
 77     finout.read((char *) &pl, sizeof pl);     //输出之前要先打开文件
 78     cout << "Your selection:\n";
 79     cout << rec << ": " << setw(LIM) << pl.name << ": "
 80          << setprecision(0) << setw(12) << pl.population
 81          << setprecision(2) << setw(6) << pl.g << endl;
 82     if (finout.eof() )
 83         finout.clear();     // clear eof flag
 84 
 85     cout << "Enter planet name: ";
 86     cin.get(pl.name, LIM);              //读入字符：cin.get(Name,sizeof(Type) ) 
 87     eatline();
 88     cout << "Enter planetary population: ";
 89     cin >> pl.population;
 90     eatline();
 91     cout << "Enter planet‘s acceleration of gravity: ";
 92     cin >> pl.g;
 93     finout.seekp(place);    // 对文件进行操作后指针会变化 重新定位指针
 94     finout.write((char *) &pl, sizeof pl) << flush;  //防止程序中断来不及送出数据   <<flush
 95     if (finout.fail())
 96     {
 97         cerr << "Error on attempted write\n";
 98         exit(EXIT_FAILURE);
 99     }
100 
101 // show revised file
102     ct = 0;
103     finout.seekg(0);            // 接下来输出全部内容，将指针定位与开始位置
104     cout << "Here are the new contents of the " << file
105          << " file:\n";
106     while (finout.read((char *) &pl, sizeof pl))
107     {
108         cout << ct++ << ": " << setw(LIM) << pl.name << ": "
109              << setprecision(0) << setw(12) << pl.population
110              << setprecision(2) << setw(6) << pl.g << endl;
111     }
112     finout.close();      //关闭文件
113     cout << "Done.\n";
114 // keeping output window open
115     // cin.clear();
116     // eatline();
117     // cin.get();
118     return 0;
119 }