Read file into Pascal AnsiString efficiently

Question

I have this code to read file contents to an AnsiString variable.

var
    c:char;
    f:file of char;
    s:ansistring;
begin
    assign(f,'file');
    reset(f);
        s:='';
        while not eof(f) do
            begin
                read(f,c);
                s:=s+c;
            end;
    close(f);
end;

This code runs very slowly. I have a file 1 MB and the program runs for about 27 seconds.

How do I read file contents to AnsiString faster?

Answer 1

        begin
            read(f,c);
            s:=s+c;
        end;

You are reading character/character and appending in string which is why your program is slow. It is not logical to read entire file in single variable. Use buffer to store the content of file read then process it and free the buffer for next read input.

program ReadFile;

uses
 Sysutils, Classes;

const
  C_FNAME = 'C:\textfile.txt';

var
  tfIn: TextFile;
  s: string;
  Temp : TStringList;
begin
  // Give some feedback
  writeln('Reading the contents of file: ', C_FNAME);
  writeln('=========================================');
  Temp := TStringList.Create;
  // Set the name of the file that will be read
  AssignFile(tfIn, C_FNAME);

  // Embed the file handling in a try/except block to handle errors gracefully
  try
    // Open the file for reading
    reset(tfIn);

    // Keep reading lines until the end of the file is reached
    while not eof(tfIn) do
    begin
      readln(tfIn, s);
      Temp.Append(s);
    end;

    // Done so close the file
    CloseFile(tfIn);

    writeln(temp.Text);
  except
    on E: EInOutError do
     writeln('File handling error occurred. Details: ', E.Message);
  end;


  //done clear the TStringList
   temp.Clear;
   temp.Free;   

  // Wait for the user to end the program
  writeln('=========================================');
  writeln('File ', C_FNAME, ' was probably read. Press enter to stop.');
  readln;
end.   

More examples at File Handling in Pascal