C++/File/fstream

Demonstrate File random access.

<source lang="cpp">

1. include <iostream>
2. include <fstream>
3. include <cstdlib>

using namespace std;

int main(int argc, char *argv[]) {

 if(argc != 3) { 
   cout << "Usage: CHANGE <filename> <byte>\n"; 
   return 1; 
 } 

 fstream out(argv[1], ios::in | ios::out | ios::binary); 
 if(!out) { 
   cout << "Cannot open file.\n"; 
   return 1; 
 } 

 out.seekp(atoi(argv[2]), ios::beg); 

 out.put("X"); 
 out.close(); 

 return 0; 

}

 </source>

File Stream Objects as Function Arguments

<source lang="cpp">

1. include <fstream>
2. include <iostream>
3. include <string>

using namespace std; bool writeFile (ofstream&, char*); bool readFile (ifstream&, char*); int main () {

  string data;
  bool status;
  ofstream outfile;
  status = writeFile(outfile, "students.dat");
  if (!status)
  {
     cout << "File could not be opened for writing\n";
     cout << "Program terminating\n";
     return 0;
  }
  else
  {
     cout << "Writing to the file" << endl;
     cout << "Enter class name: "; 
     getline(cin, data); 
     outfile << data<< endl;
     cout << "Enter number of students: "; 
     cin >> data;
     cin.ignore();
     outfile << data<< endl;
     outfile.close();
  }
  ifstream infile;
  status = readFile(infile, "students.dat");
  if (!status)
  {
     cout << "File could not be opened for reading\n";
     cout << "Program terminating\n";
     return 0;
  }
  else
  {
     cout << "Reading from the file" << endl; 
     getline(infile, data);
     while(!infile.fail())
     {
        cout << data << endl; 
        getline(infile, data);
     }
     infile.close();
  }
  return 0;

} bool writeFile (ofstream& file, char* strFile) {

  file.open(strFile);
  if (file.fail())
        return false;
  else
        return true;

} bool readFile (ifstream& ifile, char* strFile) {

  ifile.open(strFile);
  if (ifile.fail())
        return false;
  else
        return true;

}

 </source>

fstream seekp: Seek file pointer position

<source lang="cpp">

1. include <iostream>
2. include <fstream>
3. include <cstdlib>

using namespace std; int main(int argc, char *argv[]) {

 if( argc != 4) {
   cout << "Usage: CHANGE <filename> <character> <char>\n";
   return 1;
 }
 fstream out(argv[1], ios::in | ios::out | ios::binary);
 if(!out) {
   cout << "Cannot open file.";
   return 1;
 }
 out.seekp(atoi(argv[ 2 ]), ios::beg);
 out.put(*argv[ 3 ]);
 out.close();
 return 0;

}

 </source>

Reverse file content

<source lang="cpp">

1. include <iostream>
2. include <fstream>
3. include <cstdlib>

using namespace std; int main(int argc, char *argv[]) {

 if(argc!=3) {
   cout << "Usage: Reverse <filename> <num>\n";
   return 1;
 }
 fstream inout(argv[1], ios::in | ios::out | ios::binary);
 if(!inout) {
   cout << "Cannot open input file.\n";
   return 1;
 }
 long e, i, j;
 char c1, c2;
 e = atol(argv[2]);
 for(i = 0, j = e; i < j; i++, j--) {
   inout.seekg(i, ios::beg);
   inout.get(c1);
   inout.seekg(j, ios::beg);
   inout.get(c2);
   inout.seekp(i, ios::beg);
   inout.put(c2);
   inout.seekp(j, ios::beg);
   inout.put(c1);
 }
 inout.close();
 return 0;

}

 </source>

Reverses the first N characters within a file

<source lang="cpp">

1. include <iostream>
2. include <fstream>
3. include <cstdlib>

using namespace std; int main(int argc, char *argv[]) {

 long n, i, j;
 char ch1, ch2;
 if(argc!=3) {
   cout << "Usage: Reverse <filename> <num>\n";
   return 1;
 }
 fstream finout(argv[1], ios::in | ios::out | ios::binary);
 if(!finout) {
   cout << "Cannot open input file.\n";
   return 1;
 }
 n = atol(argv[2]) - 1;
 for(i=0, j=n; i < j; ++i, --j) {
   finout.seekg(i, ios::beg);
   finout.get(ch1);

   finout.seekp(j, ios::beg);
   finout.put(ch1);
   if(!finout.good()) {
     cout << "Error reading or writing characters.";
     finout.clear();
     break;
   }
 }
 finout.close();
 if(!finout.good()) {
   cout << "A file error occurred.";
   return 1;
 }
 return 0;

}

 </source>

To read or write to a file, you include <fstream>

<source lang="cpp">

1. include <fstream>

int main() {

 using namespace std;
 int intValue;
 float realValue;
 ifstream inData;
 ofstream outData;
 inData.open("input.dat");
 outData.open("output.dat");
 inData  >> intValue;
 inData  >> realValue;
 outData << "The input values are "
         << intValue  << " and "
         << realValue   << endl;
 return 0 ;

}

 </source>