Modifying or deleting a line from a text file the low-level way?

Question

I\'m working with a Text File in Delphi, and I don\'t wish to use the method of loading/saving with a string list. I intend to maintain an open filestream where I read and w

Answer 1

Without loading the entire file into a container like TStringList, your only option is to:

• Open the file for input
• Open a separate copy for output
• Start a loop
• Read the content line by line from the input file
• Write the content out line by line to the output file until you reach the line you want to change/delete
• Break the loop
• Read the input line from the input file
• Write the changed line (or skip writing the line you want to delete) to the output file
• Start a new loop
• Read the remainder of the input content, line by line
• Write the rest of that input to the output file, line by line
• Break the loop
• Close the files

So to answer your specific questions:

if N = UpperCase(Name) then begin
  //How to re-write this line?
  Break;
end;

WriteLn the new output to the second (output) file.

if N = UpperCase(Name) then begin
  //How to delete this line?
  Break;
end;

Just skip the WriteLn that outputs the indicated line to the second (output) file.

Your artificial limitation of "I don't want to use TStringList" simply complicates the task for you, when you can simply:

• Load the original file into TStringList using LoadFromFile
• Locate the line you want to modify, either by index, iteration, or IndexOf()
• Modify the line by changing it directly, or deleting it from the TStringList
• Write the entire content out to the original file using TStringList.SaveToFile

The only reasons I've found to not use TStringList to perform these kinds of operations have been that the file size exceeds the capacity of a TStringList (never happened) or when dealing with a file that is text but isn't really "line" oriented (for instance, EDI files that are typically one very long single line of text, or XML files that may not contain line feeds and therefore are also one very long single line of text). Even in the case of EDI or XML, though, it's quite frequently to load them into a TStringList, make the conversion to line-based format (inserting line breaks or whatever), and do the retrieval from the stringlist.

Answer 2

Basically, you can't do what you want to do if you treat the files as simple text files. Such files can be read (from the beginning only) or written to (either from the start, thus creating a new file) or from the end (appending to an existing file). They are not random access files.

On the other hand, you might want to consider defining a file of type string: each record in the file would be a string, and you can access this file in a random fashion. The problem then becomes in knowing which record to access for which string.

A third possibility is using INI files which are more structured and sound like a better bet for your purposes. Apart from the section header, they are a series of strings, key=value, and can be accessed on the basis of the key.

Answer 3

I find this an interesting question, so I made a small console app.

I used 3 methods:

• TStringList
• Streamreader/StreamWriter
• Text file

All methods are timed and repeated 100 times with a text file of 10kb in size and a text file 1Mb in size. Here is the program:

program Project16;

{$APPTYPE CONSOLE}

uses
  SysUtils, Classes, StrUtils, Diagnostics, IOUtils;

procedure DeleteLine(StrList: TStringList; SearchPattern: String);

var
  Index : Integer;

begin
 for Index := 0 to StrList.Count-1 do
  begin
   if ContainsText(StrList[Index], SearchPattern) then
    begin
     StrList.Delete(Index);
     Break;
    end;
  end;
end;

procedure DeleteLineWithStringList(Filename : string; SearchPattern : String);

var StrList : TStringList;

begin
 StrList := TStringList.Create;
 try
  StrList.LoadFromFile(Filename);
  DeleteLine(StrList, SearchPattern);
  // don't overwrite our input file so we can test
  StrList.SaveToFile(TPath.ChangeExtension(Filename, '.new'));
 finally
  StrList.Free;
 end;
end;

procedure DeleteLineWithStreamReaderAndWriter(Filename : string; SearchPattern : String);

var
  Reader    : TStreamReader;
  Writer    : TStreamWriter;
  Line      : String;
  DoSearch  : Boolean;
  DoWrite   : Boolean;

begin
 Reader := TStreamReader.Create(Filename);
 Writer := TStreamWriter.Create(TPath.ChangeExtension(Filename, '.new'));
 try
  DoSearch := True;
  DoWrite := True;
  while Reader.Peek >= 0 do
   begin
    Line := Reader.ReadLine;
    if DoSearch then
     begin
      DoSearch := not ContainsText(Line, SearchPattern);
      DoWrite := DoSearch;
     end;
    if DoWrite then
     Writer.WriteLine(Line)
    else
     DoWrite := True;
   end;
 finally
  Reader.Free;
  Writer.Free;
 end;
end;

procedure DeleteLineWithTextFile(Filename : string; SearchPattern : String);

var
 InFile    : TextFile;
 OutFile   : TextFile;
 Line      : String;
 DoSearch  : Boolean;
 DoWrite   : Boolean;


begin
 AssignFile(InFile, Filename);
 AssignFile(OutFile, TPath.ChangeExtension(Filename, '.new'));
 Reset(InFile);
 Rewrite(OutFile);
 try
  DoSearch := True;
  DoWrite := True;
  while not EOF(InFile) do
   begin
    Readln(InFile, Line);
    if DoSearch then
     begin
      DoSearch := not ContainsText(Line, SearchPattern);
      DoWrite := DoSearch;
     end;
    if DoWrite then
     Writeln(OutFile, Line)
    else
     DoWrite := True;
   end;
 finally
  CloseFile(InFile);
  CloseFile(OutFile);
 end;
end;

procedure TimeDeleteLineWithStreamReaderAndWriter(Iterations : Integer);

var
  Count : Integer;
  Sw    : TStopWatch;

begin
 Writeln(Format('Delete line with stream reader/writer - file 10kb, %d iterations', [Iterations]));
 Sw := TStopwatch.StartNew;
 for Count := 1 to Iterations do
  DeleteLineWithStreamReaderAndWriter('c:\temp\text10kb.txt', 'thislinewillbedeleted=');
 Sw.Stop;
 Writeln(Format('Elapsed time : %d milliseconds', [Sw.ElapsedMilliseconds]));
 Writeln(Format('Delete line with stream reader/writer - file 1Mb, %d iterations', [Iterations]));
 Sw := TStopwatch.StartNew;
 for Count := 1 to Iterations do
  DeleteLineWithStreamReaderAndWriter('c:\temp\text1Mb.txt', 'thislinewillbedeleted=');
 Sw.Stop;
 Writeln(Format('Elapsed time : %d milliseconds', [Sw.ElapsedMilliseconds]));
end;

procedure TimeDeleteLineWithStringList(Iterations : Integer);

var
  Count : Integer;
  Sw    : TStopWatch;

begin
 Writeln(Format('Delete line with TStringlist - file 10kb, %d iterations', [Iterations]));
 Sw := TStopwatch.StartNew;
 for Count := 1 to Iterations do
  DeleteLineWithStringList('c:\temp\text10kb.txt', 'thislinewillbedeleted=');
 Sw.Stop;
 Writeln(Format('Elapsed time : %d milliseconds', [Sw.ElapsedMilliseconds]));
 Writeln(Format('Delete line with TStringlist - file 1Mb, %d iterations', [Iterations]));
 Sw := TStopwatch.StartNew;
 for Count := 1 to Iterations do
  DeleteLineWithStringList('c:\temp\text1Mb.txt', 'thislinewillbedeleted=');
 Sw.Stop;
 Writeln(Format('Elapsed time : %d milliseconds', [Sw.ElapsedMilliseconds]));
end;

procedure TimeDeleteLineWithTextFile(Iterations : Integer);

var
  Count : Integer;
  Sw    : TStopWatch;

begin
 Writeln(Format('Delete line with text file - file 10kb, %d iterations', [Iterations]));
 Sw := TStopwatch.StartNew;
 for Count := 1 to Iterations do
  DeleteLineWithTextFile('c:\temp\text10kb.txt', 'thislinewillbedeleted=');
 Sw.Stop;
 Writeln(Format('Elapsed time : %d milliseconds', [Sw.ElapsedMilliseconds]));
 Writeln(Format('Delete line with text file - file 1Mb, %d iterations', [Iterations]));
 Sw := TStopwatch.StartNew;
 for Count := 1 to Iterations do
  DeleteLineWithTextFile('c:\temp\text1Mb.txt', 'thislinewillbedeleted=');
 Sw.Stop;
 Writeln(Format('Elapsed time : %d milliseconds', [Sw.ElapsedMilliseconds]));
end;

begin
  try
    TimeDeleteLineWithStringList(100);
    TimeDeleteLineWithStreamReaderAndWriter(100);
    TimeDeleteLineWithTextFile(100);
    Writeln('Press ENTER to quit');
    Readln;
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

Output:

Delete line with TStringlist - file 10kb, 100 iterations
Elapsed time : 188 milliseconds
Delete line with TStringlist - file 1Mb, 100 iterations
Elapsed time : 5137 milliseconds
Delete line with stream reader/writer - file 10kb, 100 iterations
Elapsed time : 456 milliseconds
Delete line with stream reader/writer - file 1Mb, 100 iterations
Elapsed time : 22382 milliseconds
Delete line with text file - file 10kb, 100 iterations
Elapsed time : 250 milliseconds
Delete line with text file - file 1Mb, 100 iterations
Elapsed time : 9656 milliseconds
Press ENTER to quit

As you can see is TStringList the winner here. Since you are not able to use TStringList, TextFile is not a bad choice after all...

P.S. : this code omits the part where you have to delete the inputfile and rename the outputfile to the original filename