114 lines
3.4 KiB
C++
114 lines
3.4 KiB
C++
// random.cpp -- random access to a binary file
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#include <iostream> // not required by most systems
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#include <fstream>
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#include <iomanip>
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#include <cstdlib> // (or stdlib.h) for exit()
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const int LIM = 20;
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struct planet
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{
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char name[LIM]; // name of planet
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double population; // its population
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double g; // its acceleration of gravity
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};
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const char * file = "planets.dat"; // ASSUMED TO EXIST (binary.cpp example)
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inline void eatline() { while (std::cin.get() != '\n') continue; }
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int main()
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{
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using namespace std;
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planet pl;
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cout << fixed;
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// show initial contents
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fstream finout; // read and write streams
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finout.open(file,
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ios_base::in | ios_base::out | ios_base::binary);
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//NOTE: Some Unix systems require omitting | ios::binary
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int ct = 0;
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if (finout.is_open())
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{
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finout.seekg(0); // go to beginning
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cout << "Here are the current contents of the "
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<< file << " file:\n";
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while (finout.read((char *) &pl, sizeof pl))
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{
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cout << ct++ << ": " << setw(LIM) << pl.name << ": "
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<< setprecision(0) << setw(12) << pl.population
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<< setprecision(2) << setw(6) << pl.g << endl;
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}
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if (finout.eof())
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finout.clear(); // clear eof flag
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else
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{
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cerr << "Error in reading " << file << ".\n";
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exit(EXIT_FAILURE);
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}
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}
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else
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{
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cerr << file << " could not be opened -- bye.\n";
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exit(EXIT_FAILURE);
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}
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// change a record
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cout << "Enter the record number you wish to change: ";
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long rec;
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cin >> rec;
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eatline(); // get rid of newline
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if (rec < 0 || rec >= ct)
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{
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cerr << "Invalid record number -- bye\n";
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exit(EXIT_FAILURE);
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}
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streampos place = rec * sizeof pl; // convert to streampos type
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finout.seekg(place); // random access
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if (finout.fail())
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{
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cerr << "Error on attempted seek\n";
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exit(EXIT_FAILURE);
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}
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finout.read((char *) &pl, sizeof pl);
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cout << "Your selection:\n";
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cout << rec << ": " << setw(LIM) << pl.name << ": "
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<< setprecision(0) << setw(12) << pl.population
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<< setprecision(2) << setw(6) << pl.g << endl;
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if (finout.eof())
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finout.clear(); // clear eof flag
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cout << "Enter planet name: ";
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cin.get(pl.name, LIM);
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eatline();
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cout << "Enter planetary population: ";
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cin >> pl.population;
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cout << "Enter planet's acceleration of gravity: ";
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cin >> pl.g;
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finout.seekp(place); // go back
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finout.write((char *) &pl, sizeof pl) << flush;
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if (finout.fail())
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{
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cerr << "Error on attempted write\n";
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exit(EXIT_FAILURE);
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}
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// show revised file
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ct = 0;
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finout.seekg(0); // go to beginning of file
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cout << "Here are the new contents of the " << file
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<< " file:\n";
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while (finout.read((char *) &pl, sizeof pl))
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{
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cout << ct++ << ": " << setw(LIM) << pl.name << ": "
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<< setprecision(0) << setw(12) << pl.population
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<< setprecision(2) << setw(6) << pl.g << endl;
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}
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finout.close();
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cout << "Done.\n";
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// keeping output window open
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// cin.clear();
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// eatline();
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// cin.get();
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return 0;
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}
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