Text file modifying

This is a discussion on Text file modifying within the C++ Programming forums, part of the General Programming Boards category; Hello there, I have a problem of modifying a text file with C++. Here is a text file "show.txt" that ...

  1. #1
    Registered User
    Join Date
    Oct 2008
    Posts
    12

    Text file modifying

    Hello there,

    I have a problem of modifying a text file with C++.


    Here is a text file "show.txt" that I want to modify.
    Each line has one record and Each record is separated by '\n'.
    And I want to correct 'c' of the movie title on the third line. ('c'ack to the Future)
    -----------------------------------------------------------------
    S001|terminator2|2009-10-29|02:10|X|0\n
    S002|beautiful life|2009-10-30|02:11|P|1\n
    S003|cack to the Future|2009-10-31|02:12|R|2\n
    S004|Shindler List|2009-11-01|02:13|X|3\n
    S005|the Matrix|2009-11-02|02:14|P|4\n
    S006|Titanic|2009-11-03|02:15|R|5\n



    Code:
    int main(int argc, _TCHAR* argv[])
    {
    	fstream file;
    	char str[100];
    	streamoff size;
    
    	file.open("show.txt", ios::in);
    	file.getline(str, 50, '\n');
    	file.getline(str, 50, '\n');
    	size=file.tellp(); // I have the position of "cack to the Future" record.
    	file.close();
    	
    	size+=5;// to point out the first letter of the movie title, 'c'
    	
    	file.open("show.txt", ios::out);
    	file.seekp(size, ios::beg);
    	file.write("b",1); //try to overwrite from "c" to "b"
    	file.close();
    
                    return 0;
    }

    I got the result bellow. (I just wanted to switch a character but everything has gone but 'b', it seems like that I have tried to modify.)
    -----------------------------------------------------------------

    b


    Can anyone give me an advice?
    Thank you.
    Last edited by a0161; 10-30-2009 at 06:24 AM.

  2. #2
    Registered User whiteflags's Avatar
    Join Date
    Apr 2006
    Location
    United States
    Posts
    7,762
    C++ doesn't offer that kind of control. Usually you are forced to rewrite the entire file after changing it in RAM. Here's a minimal example of the way:

    Code:
    #include <string>
    #include <fstream>
    #include <cstdio>
    
    int main()
    {
       // declarations here
    
       while(std::getline(infile, str)) {
          std::string::size_type pos = str.find("cack to the future");
          if(pos != std::string::npos) {
             str[pos] = 'b';
             outfile << str << '\n';
          }
          else {
             outfile << str << '\n';
          }
       }
       infile.close();
       outfile.close();
       std::remove("show.txt");
       std::rename("show.temp", "show.txt");
    
       return 0;
    }
    Lots of fancier ways I suppose though. And you'll want to make sure that all file changes like rename and remove were successful.
    Last edited by whiteflags; 10-30-2009 at 06:55 AM.

  3. #3
    Registered User
    Join Date
    Oct 2008
    Posts
    12

    Thank you.

    Thank you for your help.
    It's very useful lesson for me.!

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