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

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