random.cpp

// rnadom.cpp 随机访问二进制文件

#include <iostream>
#include <fstream>
#include <iomanip>
#include <cstdlib>

const int LIM = 20;

struct planet
{
    char name[LIM];     // name of planet
    double population;  // its population
    double g;           // its acceleration of gravity
};

const char * file = "planets.dat"; // assumed to exist
inline void eatline() {while (std::cin.get() != '\n') continue;}

int main()
{
    using namespace std;
    planet pl;
    cout << fixed;

    // show initial contents
    fstream finout; // read and write streams
    finout.open(file, ios_base::in | ios_base::out | ios_base::binary);

    // NOTES : Some Unix Sytems require omitting | ios_base::binary
    int ct = 0;
    if (finout.is_open())
    {
        finout.seekg(0); // go to beginning
        cout << " Here are the current contents of the " 
             << file << " file : \n";
        while (finout.read((char *) &pl, sizeof pl));
        cout << ct++ << " : " << setw(LIM) << pl.name << " : "
             << setprecision(0) << setw(LIM) << pl.population
             << setprecision(2) << setw(6) << pl.g << endl;
        if (finout.eof())
            finout.clear(); // clear of flag
        else
        {
            cerr << "Error in Reading " << file << ".\n";
            exit(EXIT_FAILURE);
        }
    }
    else
    {
        cerr << file << "could not be opened ----- bye .\n";
        exit(EXIT_FAILURE);
    }

    // change a record
    cout << "Enter the record number you wish to change : ";
    long rec;
    cin >> rec;
    eatline();
    if (rec < 0 || rec >= ct)
    {
        cerr << "Invalid record number ---- bye \n";
        exit(EXIT_FAILURE);
    }
    streampos place = rec * sizeof pl; // 转换为 streampos type
    finout.seekg(place); // random access
    if (finout.fail())
    {
        cerr << " Error on attemped seek \n";
        exit(EXIT_FAILURE);
    }
    finout.read((char *) &pl, sizeof pl);
    cout << "Your selection:\n";
    cout << rec << ":" << setw(LIM) << pl.name << ":"
         << setprecision(0) << setw(12) << pl.population
         << setprecision(2) << setw(6) << pl.g << endl;
    if (finout.eof())
        finout.clear(); // clear of flag
    
    cout << "Enter planet name : ";
    cin.get(pl.name,LIM);
    eatline();
    cout << "Enter planetary population : ";
    cin >> pl.population;
    cout << "Enter planet's acceleration of gravity : ";
    cin >> pl.g;
    finout.seekg(place); // go back
    finout.write((char *) &pl,sizeof pl) << flush;
    if(finout.fail())
    {
        cerr << "Error on attempted write \n";
        exit(EXIT_FAILURE);
    }

    // show revised file
    ct = 0;
    finout.seekg(0);
    cout << "Here are the new contents of the " << file << " file; \n";

    while (finout.read((char *) &pl, sizeof pl))
    {
        cout << ct++ << " : " << setw(LIM) << pl.name << ": "
             << setprecision(0) << setw(12) << pl.population
             << setprecision(2) << setw(6) << pl.g << endl;
    }
    finout.close();
    cout << "Done. \n";
    return 0;
}