Question

Replace string in large text files

Asked by: Tomazaz

Hello,

Please suggest any solution how to replace string in large files(~20mb) using .NET?

Regards,
Tomas

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Asked On
2009-02-03 at 23:58:45ID24111024
Topic

.NET

Participating Experts
3
Points
125
Comments
12

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Answers

 

by: isaackhaziPosted on 2009-02-04 at 00:01:09ID: 23545759

 

by: isaackhaziPosted on 2009-02-04 at 00:02:36ID: 23545765

 

by: TomazazPosted on 2009-02-04 at 00:18:50ID: 23545826

This is code for c++ not for .NET http://www.codeproject.com/KB/files/scanr.aspx

 

by: TomazazPosted on 2009-02-04 at 00:20:45ID: 23545832

This is also not native NET code, it use VB scripting.

<<This one also:

http://www.motobit.com/tips/detpg_replfile/>>

 

by: GreenGhostPosted on 2009-02-04 at 00:33:19ID: 23545881

Well, a 20MB file is not too large to handle in memory, unless you do it frequently:

File.WriteAllText(fileName, File.ReadAllText(fileName).Replace("find this", "replaced by this"));

If what you replace doesn't contain line breaks, you can use a temporary file and replace one line at a time:

string tempName = fileName + ".temp";
File.Move(fileName, tempName);
using (StreamReader reader = New StreamReader(tempName)) {
   using (StreamWriter writer = new StreamWriter(fileName)) {
      string line;
      while ((line = reader.ReadLine()) != null) {
         writer.WriteLine(line.Replace("find this", "replaced by this"));
      }
   }
}
file.Delete(tempName);

 

by: TomazazPosted on 2009-02-04 at 03:55:21ID: 23547012

I want to clarify my requirements, I need to  replace string in binary file. The  string is the same lenght but located in diferent places depend on file and files are large, about 20mb. So I can't use function which load a whole file to memory. I have browsed .Net functions related to file manipulation
but didn't found any usefull.

 

by: GreenGhostPosted on 2009-02-04 at 18:45:36ID: 23556020

You can't put a string in a file, you have to encode it into bytes first. Then you can use the Seek and Write methods to overwrite part of the file:

string dataString = "Something to put in the file";
int pos = 1234;
 
byte[] data = Encoding.ASCII.GetBytes(dataString);
using (FileStream s = new FileStream(fileName, FileMode.Open, FileAccess.ReadWrite)) {
   s.Seek(pos, SeekOrigin.Begin);
   s.Write(data, 0, data.Length);
}
                                              
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Select allOpen in new window

 

by: TomazazPosted on 2009-02-04 at 23:04:32ID: 23557103

There is one problem, I don't know offset of string which I need to replace, the place and file size will differ from file to file.

I have searched the whole SDK but can't find anything like SearchForOffset(Array of Bytes): Integer

 

by: DropZonePosted on 2009-02-07 at 11:59:46ID: 23579922

You can't find it because it is not there.  There is no bulit-in way to scan a file for a particular binary sequence.  That said, scanning the file manually, byte by byte, is not such a hard thing to do.  You can prefetch chunks of it and perform the byte scan in memory on each chunk, which should be faster.

A word of caution: when replacing the string, you must make sure that the new string is of the same length, otherwise may mangle the file.  If the string with which you are replacing is of a different size, then I suggest opening a temporary output file and write to it everything you read from the original file until the string you found; then write the replacement string, followed by the rest of the original file.  When done, rename the temporary file to replace the original.

      -dZ.

 

by: GreenGhostPosted on 2009-07-17 at 11:38:15ID: 24881474

I suggest a split between #23556020 and #23579922, they give a good starting point on how to work with binary files.

20120131-EE-VQP-002

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