C++ Tutorial/File Stream/ifstream

Display a file from a given starting point

<source lang="cpp">#include <iostream>

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

using namespace std;

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

 char ch; 

 ifstream in("test.txt", ios::in | ios::binary); 
 if(!in) { 
   cout << "Cannot open file.\n"; 
   return 1; 
 } 

 in.seekg(1, ios::beg); 

 while(in.get(ch)) 
   cout << ch; 

 return 0; 

}</source>

0 123.23
This is a text.

Display a file using ifstream.get()

<source lang="cpp">#include <iostream>

1. include <fstream>

using namespace std;

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

 char ch; 

 ifstream in("test", ios::in | ios::binary); 
 if(!in) { 
   cout << "Cannot open file.\n"; 
   return 1; 
 } 

 while(in) { // in will be false when eof is reached 
   in.get(ch); 
   if(in) cout << ch; 
 } 

 in.close(); 

 return 0; 

}</source>

hello

ifstream: read buffer and seek file position

<source lang="cpp">/* The following code example is taken from the book

* "The C++ Standard Library - A Tutorial and Reference"
* by Nicolai M. Josuttis, Addison-Wesley, 1999
*
* (C) Copyright Nicolai M. Josuttis 1999.
* Permission to copy, use, modify, sell and distribute this software
* is granted provided this copyright notice appears in all copies.
* This software is provided "as is" without express or implied
* warranty, and with no claim as to its suitability for any purpose.
*/

// header files for file I/O

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

void printFileTwice (const char* filename) {

   // open file
   std::ifstream file(filename);
   // print contents the first time
   std::cout << file.rdbuf();
   // seek to the beginning
   file.seekg(0);
   // print contents the second time
   std::cout << file.rdbuf();

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

   // print all files passed as a command-line argument twice
   for (int i=1; i<argc; ++i) {
       printFileTwice(argv[i]);
   }

}</source>

Ignore up to 10 characters or until first space is found

<source lang="cpp">#include <iostream>

1. include <fstream>

using namespace std; int main() {

 ifstream in("test");
 if(!in) {
   cout << "Cannot open file.\n";
   return 1;
 }
 in.ignore(10, " ");
 char c;
 while(in) {
   in.get(c);
   if(in) cout << c;
 }
 in.close();
 return 0;

}</source>

Read and display a text file line by line.

<source lang="cpp">#include <iostream>

1. include <fstream>

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

 ifstream in("test.txt");
 if(!in) {
   cout << "Cannot open input file.\n";
   return 1;
 }
 char str[255];
 while(in) {
   in.getline(str, 255);  // delim defaults to "\n"
   if(in) cout << str << endl;
 }
 in.close();
 return 0;

}</source>

R 9.9
T 9.9
M 4.8

12. 8. 5. Reading a text file

   <source lang="cpp">

1. include <iostream.h>
2. include <fstream.h>
3. include <stdlib.h>

int main () {

 char buffer[256];

 ifstream examplefile ("test.txt");

 if (! examplefile.is_open())
 { 
    cout << "Error opening file"; 
    exit (1); 
 }
 while (! examplefile.eof() )
 {
   examplefile.getline (buffer,100);
   cout << buffer << endl;
 }
 return 0;

}</source>

This is a line.
This is another line.

Reading numbers from a file

<source lang="cpp">#include <fstream>

1. include <iostream>
2. include <iomanip>

using std::cout; using std::endl;

int main() {

 const char* filename = "test.txt";
 std::ifstream inFile(filename);
 // Make sure the file stream is good
 if(!inFile) {
   cout << endl << "Failed to open file " << filename;
   return 1;
 }
 long n = 0;
 while(!inFile.eof()) {
   inFile >> n;
   cout << std::setw(10) << n;
 }
 cout << endl;
 return 0;

}</source>

0         1         2         3         4         5         6         7         8         9        10        11        12        13
        14        15        16        17        18        19        20        21        22        23        24        25        26        27
        28        29        30        31        32        33        34        35        36        37        38        39        40        41
        42        43        44        45        46        47        48        49        50        51        52        53        54        55
        56        57        58        59        60        61        62        63        64        65        66        67        68        69
        70        71        72        73        74        75        76        77        78        79        80        81        82        83
        84        85        86        87        88        89        90        91        92        93        94        95        96        97
        98        99

Show a file from starting offset

<source lang="cpp">#include <iostream>

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

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

 char ch;
 ifstream in("test.txt", ios::in | ios::binary);
 
 if(!in) {
   cout << "Cannot open file.";
   return 1;
 }
 in.seekg(2, ios::beg);
 while(in.get(ch))
   cout << ch;
 return 0;

}</source>

9.9
T 9.9a
M 4.8

Use ifstream.read() and ofstream.write()

<source lang="cpp">#include <iostream>

1. include <fstream>

using namespace std;

int main() {

 int n[5] = {1, 2, 3, 4, 5}; 
 register int i; 

 ofstream out("test", ios::out | ios::binary); 
 if(!out) { 
   cout << "Cannot open file.\n"; 
   return 1; 
  } 

 out.write((char *) &n, sizeof n); 

 out.close(); 

 for(i=0; i<5; i++) // clear array 
   n[i] = 0; 

 ifstream in("test", ios::in | ios::binary); 
 if(!in) { 
   cout << "Cannot open file.\n"; 
   return 1; 
 } 

 in.read((char *) &n, sizeof n); 

 for(i=0; i<5; i++) // show values read from file 
   cout << n[i] << " "; 

 in.close(); 

 return 0; 

}</source>

1 2 3 4 5