Cannot remove `Null Characters` from a string

Question

I asked a similar question couple months ago. Thanks to Rob Kennedy I could load my whole text into a Richedit BUT I couldn't remove Null chars. I could load my text because I used Stream.

---

Now in this code:

var
  strm : TMemorystream;
  str  : UTF8string;
  ss   : TStringstream;

begin
  strm := tmemorystream.Create;

  try
    strm.LoadFromFile('C:\Text.txt');
    setstring(str,PAnsichar(strm.Memory),strm.Size);
    str := StringReplace(str, #0, '', [rfReplaceAll]);  //This line doesn't work at all
    ss  := tstringstream.Create(str);
    Richedit1.Lines.LoadFromStream(ss);
  finally
    strm.Free;
    ss.Free;
  end;
end;

I converted TMemorystream to string to remove Null Chars with StringReplace() and then converted it again to TStringstream to load it with Richedit.lines.LoadFromStream.

But my problem is that I can't remove Null Character using StringReplace(). I can replace other characters but not #0.

Is there any way to remove null charcters directly in TMemorystream and load it into a Richedit? How? If it's not possible or it's very complex, how can I remove them when I convert my text to string?

Thanks.

Answer 1

As far as I can see, all searching/replacing utilities, at one time or other, cast the input to a PChar, which '#0' is the termination character. Hence they never go past the string part that's before the first Null. You may need to devise your own mechanism. Just a quick example:

var
  i: Integer;
begin
  Assert(str <> '');
  i := 1;
  while i <= Length(str) do
    if str[i] = #0 then
      Delete(str, i, 1)
    else
      Inc(i);

Replacing in the stream would similarly involve testing each character and then adjusting the stream accordingly before moving on after you decide to delete one.

Answer 2

Sertac's answer is accurate and you should accept it. If performance is important, and you have a large string with frequent instances of the null character then you should try to reduce the number of heap allocations. Here is how I would implement this:

function RemoveNull(const Input: string): string;
var
  OutputLen, Index: Integer;
  C: Char;
begin
  SetLength(Result, Length(Input));
  OutputLen := 0;
  for Index := 1 to Length(Input) do
  begin
    C := Input[Index];   
    if C <> #0 then
    begin
      inc(OutputLen);
      Result[OutputLen] := C;
    end;
  end;
  SetLength(Result, OutputLen);
end;

If you want to do it directly in the memory stream, then you can do it like this:

procedure RemoveNullFromMemoryStream(Stream: TMemoryStream);
var
  i: Integer;
  pIn, pOut: PByte;
begin
  pIn := Stream.Memory;
  pOut := pIn;
  for i := 0 to Stream.Size-1 do
  begin
    if pIn^ <> 0 then
    begin
      pOut^ := pIn^;
      inc(pOut);
    end;
    inc(pIn);
  end;
  Stream.SetSize(NativeUInt(pOut)-NativeUInt(Stream.Memory));
end;