Replace a character of the nth position

Question

I am writing this program to replace the character at the nth position of a string in a text file. My text file consists of the following contents -

the quick brown fox jumped over the lazy dog
the quick brown fox jumped over the lazy dog
the quick brown fox jumped over the lazy dog
the quick brown fox jumped over the lazy dog

And here is the output of the code -

thehuick brown fox jumped over the lazy dog

The above result is not what i wanted. Only one line is updated the rest are not found anymore within the file.

Here's my complete code in C#

        var txtFiles = Directory.GetFiles(@"E:\PROJ\replaceY\replaceY\", "*.txt");
        foreach (string currentFile in txtFiles)
        {
            string[] lines = File.ReadAllLines(currentFile);
            foreach (string line in lines)
            {

                var theString = line;
                var aStringBuilder = new StringBuilder(theString);
                aStringBuilder.Remove(3, 2);
                aStringBuilder.Insert(3, "h");
                theString = aStringBuilder.ToString();
                using (StreamWriter outfile = new StreamWriter(currentFile))
                {
                    outfile.Write(theString.ToString());
                }

                Console.WriteLine(theString);
                Console.ReadKey();

            }
        }

Where have i gone wrong? Please help!

Answer 1

Because your logic says:

• for each file
  • open the file
  • for each line in the file
    • process it
    • re-open the file for write
    • write the line

That means you repeatedly overwrite the file with the latest line. At the end of it all each file contains just one line: the last one.

You'll be wanting to do something like this:

static void Main(string[] args)
{
  foreach ( string fileName in args )
  {
    string tempFileName = Path.GetTempFileName() ;

    using ( Stream       tmpFile = File.Open( tempFileName , FileMode.OpenOrCreate , FileAccess.Write , FileShare.None ) )
    using ( StreamWriter output = new StreamWriter( tgtFile , Encoding.Default ) )
    using ( Stream       srcFile = File.Open( fileName , FileMode.Open , FileAccess.ReadWrite , FileShare.None ) )
    using ( StreamReader input = new StreamReader( srcFile , Encoding.Default , true ) )
    {
      string line ;
      while ( null != (line=input.ReadLine()) )
      {
        output.Write( line.Substring(0,3) ) ;
        output.Write( 'h' ) ;
        output.WriteLine( line.Substring(5) ) ;
      }
    }

    string backupFileName = string.Format( "{0}.{1:yyyy-MM-dd.HHmmss}.bak" , fileName , DateTime.Now ) ;

    File.Move( fileName     , backupFileName ) ;
    File.Move( tempFileName , fileName ) ;
    File.Delete( backupFileName ) ;

  }
  return;
}

You'll need to add some try/catch/finally exception handling to deal with cases where something goes south with a file, so you can roll that file back to its original state.

The other options is to

• open the file for read.
• slurp in the entire contents with File.ReadAllLines() or the equivalent
• transform the lines as you see fit
• re-open the original file for write
• spit the transformed contents out into the original file.

Which brings you to something like this:

static void Main(string[] args)
{
  foreach ( string fileName in args )
  {
    string[] lines = File.ReadAllLines( fileName ) ;

    for ( int i = 0 ; i < lines.Length ; ++i )
    {
      lines[i] = ApplyTransformHere( lines[i] ) ;
    }

    File.WriteAllLines( fileName , lines ) ;

  }

  return;
}

Answer 2

I created a boolean append at the beginning of every new file that is set to false, so it creates a new file for the first line, then sets append = true so each subsequent line is appended on

var txtFiles = Directory.GetFiles(@"E:\PROJ\replaceY\replaceY\", "*.txt");
foreach (string currentFile in txtFiles)
{
    var append = false;
    string[] lines = File.ReadAllLines(currentFile);
    foreach (string line in lines)
    {
        var theString = line;
        var aStringBuilder = new StringBuilder(theString);
        aStringBuilder.Remove(3, 2);
        aStringBuilder.Insert(3, "h");
        theString = aStringBuilder.ToString();
        using (StreamWriter outfile = new StreamWriter(currentFile, append))
        {
            outfile.Write(theString.ToString() + Environment.NewLine);
        }
        append = true;
    }
}

Answer 3

On a side note but still related. Have you consider using Regex to replace all lines?

https://regex101.com/r/aC6cY1/1

Like /(?<=^.{4,4})./gm

string input = "";//your sample text above
string pattern = "(?<=^.{4,4}).";
string replacement = "h";
Regex rgx = new Regex(pattern, RegexOptions.Multiline);
string result = rgx.Replace(input, replacement);

Regarding your issue I would:

var txtFiles = Directory.GetFiles(@"E:\PROJ\replaceY\replaceY\", "*.txt");
        foreach (string currentFile in txtFiles)
        {
        using (StreamReader sr = new StreamReader(currentFile))
            {
                 string input = sr.ReadToEnd();
                 string pattern = "(?<=^.{4,4}).";
                 string replacement = "h";
                 Regex rgx = new Regex(pattern, RegexOptions.Multiline);
                 string result = rgx.Replace(input, replacement);
                 using (StreamWriter outfile = new StreamWriter(currentFile, append))
                  {
                      outfile.Write(result);
                  }
            }
}