replace line in a file C++

Question

I\'m trying to find a way to replace a line, containing a string, in a file with a new line.

If the string is not present in the file, then append it to the file.

Answer 1

To replace the last two lines in your file:

• Use fopen() to open the file
• Get the current file position with fgetpos()
• Store always the last two file position and read line by line
• When you've reached the end of the file: The lower position is where you have to position to with fseek()
• Now you can fprintf() your new lines to the file and close it with fclose() afterwards.

Answer 2

If you want to change the length of (or delete) a line in the middle of a file, you will have to re-write all the subsequent lines.

There's no way of simply deleting or inserting characters into an existing file.

Answer 3

As mentioned, the code by KarlPhilip is a good starting point (thanks), but did not work correctly according to the ">= 0". Comparing to "0" it is known for MS CString-types and in C#, etc, but it seems not to work properly with the STL. E.g. in VS 2010 (only release), the C++ code behaved wrong with this compare without throwing an error!

Here is a correction to C++ standards: If some wizards have improvements, please post them. I think, it is a very useful and important question/snippet.

#include <iostream>
#include <fstream>
#include <string>
#include <algorithm>

using namespace std;    

int main() 
{   
   string search_string = "cactus";   
   string replace_string = "oranges";   
   string inbuf;
   // Todo: Check, if input_file exists before. 
   // Todo: Try/catch or check, if you have write access to the output file.
   fstream input_file("demo.txt", ios::in);   
   ofstream output_file("result.txt");

   while (!input_file.eof())   
   {
      getline(input_file, inbuf);
      size_t foundpos = inbuf.find(search_string);
      if(foundpos != std::string::npos)
      {
         string tmpstring = inbuf.substr(0,spot);
         tmpstring += replace_string;
         tmpstring += inbuf.substr(spot+search_string.length(), inbuf.length());
         inbuf = tmpstring;
      }

      output_file << inbuf << endl;   
   }

   //TODO: delete demo.txt and rename result.txt to demo.txt      
   // to achieve the REPLACE effect. 
}

I used the following replacement part inside the "if" from another SO question (The code above only replace the first occurrence):

if(foundpos != std::string::npos)
   replaceAll(inbuf, search_string, replace_string); 

//http://stackoverflow.com/questions/3418231/c-replace-part-of-a-string-with-another-string
//
void replaceAll(std::string& str, const std::string& from, const std::string& to) 
{
    size_t start_pos = 0;
    while((start_pos = str.find(from, start_pos)) != std::string::npos) 
    {
        size_t end_pos = start_pos + from.length();
        str.replace(start_pos, end_pos, to);
        start_pos += to.length(); // In case 'to' contains 'from', like replacing 'x' with 'yx'
    }
}

Answer 4

Although I recognize that this is not the smartest way to do it, the following code reads demo.txt line by line and searches for the word cactus to replace it for oranges while writing the output to a secondary file named result.txt.

Don't worry, I saved some work for you. Read the comment:

#include <iostream>
#include <fstream>
#include <string>
#include <algorithm>

using namespace std;


int main()
{
  string search_string = "cactus";
  string replace_string = "oranges";
  string inbuf;
  fstream input_file("demo.txt", ios::in);
  ofstream output_file("result.txt");

  while (!input_file.eof())
  {
      getline(input_file, inbuf);

      int spot = inbuf.find(search_string);
      if(spot >= 0)
      {
         string tmpstring = inbuf.substr(0,spot);
         tmpstring += replace_string;
         tmpstring += inbuf.substr(spot+search_string.length(), inbuf.length());
         inbuf = tmpstring;
      }

      output_file << inbuf << endl;
  }

  //TODO: delete demo.txt and rename result.txt to demo.txt
  // to achieve the REPLACE effect.
}