Solved

Insert a line in a text file using fstream

Posted on 2001-07-12
6
895 Views
Last Modified: 2008-02-01
Hi,

I'm new to C++. I have an application that has to insert a line in a text file (if possible without creating another file).

I have to use STL and fstream.

Any ideas how to do it?

Thanks,
Sorin
0
Comment
Question by:sorinv
  • 3
  • 2
6 Comments
 
LVL 22

Accepted Solution

by:
nietod earned 200 total points
ID: 6278332
It is impossible to insert information into a file.  you can write over information in a file, but you cannot insert new information.   i..e you can write out new information, but that new information writes on top of existig information, it does not insert the information by making room for it in the middle of the existing information.

There are two main ways to accomplish this.  

One is to copy data from the first file to a 2nd file until you reach the insert point.  then insert the new information (write it to the 2nd file), then copy the information remaining in the source file.  Then you copy all the inforamtion from the 2nd file back to the first.

The other way is to read the fle into memory, change it in memory, then write it back to disk.

example follows.
0
 
LVL 30

Expert Comment

by:Axter
ID: 6278337
You can use std::vector<string> to read each line in the text file.  While you're reading the line into the std::vector<string>, you can insert a line.
Then you can write to the text file, but writeing the std::vector<string> to it.
0
 
LVL 30

Expert Comment

by:Axter
ID: 6278349
When you read it, use the global getline function to read the text file line by line.
Something like the following:
     vector<string> lines;
     fstream in("test2.cpp");
     while (!in.eof())
     {
          string line;
          getline(in,line);
          lines.push_back(line);
     }
     in.close();
0
Is Your Active Directory as Secure as You Think?

More than 75% of all records are compromised because of the loss or theft of a privileged credential. Experts have been exploring Active Directory infrastructure to identify key threats and establish best practices for keeping data safe. Attend this month’s webinar to learn more.

 
LVL 22

Expert Comment

by:nietod
ID: 6278354
#include <fstream>
#include <vector>
#include <string>

using namespace std;

int main()
{
   vector<string> FilDat;
   fstream Fil("filename.txt");

   // Read in all the data
  while (!Fil.eof()))
  {
     string Lin;

     getline(Fil,Lin);
     FilDat.push_back(Lin);
  }

  // Insert a new line after the 3rd line.
  FilDat.insert(FilDat.begin()+2,"This is a new line");

  // Write out the data.
  const int LinCnt = FilDat.size();  

  Fil.seekp(0);
  for (int i = 0; i < LinCnt; ++i)
      Fil << FilDat[i] << endl;

   return 0;
}
0
 

Author Comment

by:sorinv
ID: 6279656
Thanks. I thought it would be this way, but I hopped you guys would have come with a better solution. Thanks nayway for clarifying the things for me.
0
 
LVL 22

Expert Comment

by:nietod
ID: 6280224
Every word processor in the world works in one of these ways.   Most are now memory based, readig all the file in and manipulating it in memory, but when memory was tight they sed to be mostly file based.
0

Featured Post

Is Your Active Directory as Secure as You Think?

More than 75% of all records are compromised because of the loss or theft of a privileged credential. Experts have been exploring Active Directory infrastructure to identify key threats and establish best practices for keeping data safe. Attend this month’s webinar to learn more.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

Title # Comments Views Activity
C++ get user from AD  (VS6) 7 55
Fast, free, private online code repository / editor 7 90
Writing a parser for java language 4 71
sorting efficency of sorting algorithm 30 103
  Included as part of the C++ Standard Template Library (STL) is a collection of generic containers. Each of these containers serves a different purpose and has different pros and cons. It is often difficult to decide which container to use and …
Container Orchestration platforms empower organizations to scale their apps at an exceptional rate. This is the reason numerous innovation-driven companies are moving apps to an appropriated datacenter wide platform that empowers them to scale at a …
The goal of the tutorial is to teach the user how to use functions in C++. The video will cover how to define functions, how to call functions and how to create functions prototypes. Microsoft Visual C++ 2010 Express will be used as a text editor an…
The goal of the video will be to teach the user the difference and consequence of passing data by value vs passing data by reference in C++. An example of passing data by value as well as an example of passing data by reference will be be given. Bot…

895 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

11 Experts available now in Live!

Get 1:1 Help Now