I have a file output.txt as follows :

p val 25 36
8 1 0
-1 -8 0
-9 -7 0
-9 -2 0
7 2 9 0
-10 6 0
-10 8 0
-6 -8 10 0
-11 -9 0
-11 -3 0
9 3 11 0
12 -10 0
12 -9 0
10 9 -12 0
13 -10 0

I wish to change the line "p val 25 36" to "p val 26 37". Is there a way to do so besides copying the file to a new file and changing the line when writing to the new file.

Is it possible to do using seekp and tellg.


8 Years
Discussion Span
Last Post by Comatose

Yes you can. You could use an fstream for example, that would be the easiest way to do it. Or you can use an ifstream reading the file, then an ofstream to modify it.

Move the pointer (seekp) to the desired location and start writing out your data. You can put out your characters one by one, using "put". This is one way to do it. Or you can use "write". The tricky part is to determinate, where the pointer should be, and of course not to overwrite your "valid" data.


#include <fstream>
using namespace std;

int main () {

  char * buffer;
  long size;

  ifstream infile ("output.txt",ifstream::binary);
  ofstream outfile ("new.txt",ofstream::binary);

  // get size of file

  // allocate memory for file content
  buffer = new char [size];

  // read content of infile
  infile.read (buffer,size);

  // write to outfile
  outfile.write (buffer,size);
  // release dynamically-allocated memory
  delete[] buffer;

  return 0;

How do i change the line ? Here is the code which copy the file but how do i change the line in the code?

#include <iostream>
#include <fstream>

using namespace std;

int main()
	fstream myfile; // fstream for reading, and writing the file
	char your_thingy[] = "p val 26 37", tmp; // i used char array to store your data
	unsigned int pos = 0; // position of "seekp" - where you want start writing

	myfile.open( "yourfile.txt", ios::binary | ios::in | ios::out  ); // using binary mode
	if( !myfile ) // exit if unable to open
		cout << "File does not exist!" << endl;
		cin.get(); // works as "pause"
	do // finding 'p' since you want to find this line "p val 25 36"
		tmp = myfile.get(); // reading by char
		pos++; // incrementing pos
	while( !myfile.eof() && tmp != 'p' ); // stop when 'p' found, or end of file

	myfile.seekp( pos-1, ios::beg ); //set pointer to 'p'
	myfile.write( your_thingy, strlen( your_thingy ) ); //write out your sentence
	// strlen gives back the size of the string

	myfile.close(); // closing file

	cout << "I am a leaf on the wind. Watch me soar. ;)" << endl; // Quote from Firefly :)
	cin.get();	// NO system( "PAUSE" ); use cin.get(); instead :)
	return 0;

It is not perfect, but works. You have to modify the code, to make it safe. ( Like what if 'p'is not in the text, but this was a specific task so I am not worried. ) You can also use different type of containers, not char *, like I did. Good luck with it.


I would just getline() each line into a vector of strings, find() the cullprit, then change it via the iterator find() returns. then output the new vector to the file. i estimate a 10 line solution or less.


Hmm, tempting maybe I write it again, based on your idea. I wonder how much resource that would use. :-/



Could you give an example of how to find and change the line?

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