C++ Primer Plus

Finally, to display the revised file, the program uses seekg() to reset the file pointer to the beginning. Listing 17.20 shows the complete program. Don’t forget that it assumes that a planets.dat file created using the binary.cpp program is available.

Note

The older the implementation, the more likely it is to run afoul of the C++ Standard. Some systems don’t recognize the binary flag, the fixed and right manipulators, and ios_base.

Listing 17.20. random.cpp

// random.cpp -- random access to a binary file

#include      // not required by most systems

#include

#include

#include       // for exit()

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 (binary.cpp example)

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);

    //NOTE: Some Unix systems require omitting | ios::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(12) << pl.population

                 << setprecision(2) << setw(6) << pl.g << endl;

        }

        if (finout.eof())

            finout.clear(); // clear eof 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();              // get rid of newline

    if (rec < 0 || rec >= ct)

    {

        cerr << "Invalid record number -- bye\n";

        exit(EXIT_FAILURE);

    }

    streampos place = rec * sizeof pl;  // convert to streampos type

    finout.seekg(place);    // random access

    if (finout.fail())

    {

        cerr << "Error on attempted 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 eof 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.seekp(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);            // go to beginning of file

    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;

}

Here’s a sample run of the program in Listing 17.20, based on a planets.dat file that has had a few more entries added since you last saw it:

Here are the current contents of the planets.dat file:

0:                Earth:   6928198253  9.81

1:        Jenny's World:     32155648  8.93

2:              Tramtor:  89000000000 15.03

3:              Trellan:      5214000  9.62

4:            Freestone:   3945851000  8.68

5:            Taanagoot:    361000004 10.23

6:                Marin:       252409  9.79

Enter the record number you wish to change: 2

Your selection:

2:              Tramtor:  89000000000 15.03

Enter planet name: Trantor

Enter planetary population: 89521844777

Enter planet's acceleration of gravity: 10.53

Here are the new contents of the planets.dat file:

0:                Earth:   6928198253  9.81

1:        Jenny's World:     32155648  8.93

2:              Trantor:  89521844777 10.53

3:              Trellan:      5214000  9.62

4:            Freestone:   3945851000  8.68

5:            Taanagoot:    361000004 10.23

6:                Marin:       252409  9.79

Done.

By using the techniques in this program, you can extend it so that it allows you to add new material and delete records. If you were to expand the program, it would be a good idea to reorganize it by using classes and functions. For example, you could convert the planet structure to a class definition; then you could overload the << insertion operator so that cout << pl displays the class data members formatted as in the example. Also the example doesn’t bother to verify input, so you could add code to check for numeric input where appropriate.

Working with Temporary Files