Using peek( ), unget( ), and ignore() to read file - C++ File Stream

C++ examples for File Stream:File Operation

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Description

Using peek( ), unget( ), and ignore() to read file

Demo Code

#include <iostream>
#include <fstream>
#include <cctype>
using namespace std;
int main()//from  w w  w  .j  av  a2  s.co  m
{
   char ch;
   char idnum[5];
   // Null-terminate idnum so that it can hold a char * string.
   idnum[4] = 0;
   // Create an ofstream object and attempt to open the file test.dat.
   ofstream fout("test.dat");
   // Verify that the file has been successfully opened.
   if(!fout) {
      cout << "Cannot open test.dat for output.\n";
      return 1;
   }
   // Write some information to the file.
   fout << "Tom #1111\nRalph #1234\nTed #2222\n";
   fout << "Holden #pending\nJones, #8888\n";
   // Close the output file.
   fout.close();
   if(!fout.good()) {
      cout << "Error creating data file.";
      return 1;
   }
   // Attempt to open the test.dat file.
   ifstream fin("test.dat");
   if(!fin) {
      cout << "Cannot open test.dat for input.\n";
      return 1;
   }
   // Use exceptions to watch for errors.
   fin.exceptions(ios::badbit | ios::failbit);
   try {
      // Find and display all ID numbers:
      do {
         // Find the start of an ID number.
         fin.ignore(40, '#');
         // If the end of the file is encountered, stop reading.
         if(fin.eof()) {
            fin.clear(); // clear eofbit
            break;
         }
         ch = fin.peek();
         if(isdigit(ch)) {
            fin.read((char *)idnum, 4);
            cout << "ID #: " << idnum << endl;
         } else {
            cout << "ID not available: ";
            ch = fin.get();
            while(isalpha(ch)) {
               cout << ch;
               ch = fin.get();
            };
            fin.unget();
            cout << endl;
         }
      } while(fin.good());
   } catch(ios_base::failure exc) {
      cout << "Error reading data file.\n";
   }
   try {
      fin.close();
   } catch (ios_base::failure exc) {
      cout << "Error closing data file.";
      return 1;
   }
   return 0;
}

Result


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