pos von hinten

[himitsu]

Zitat:

Delphi-Quellcode:

function String_Reverse(S:String):String;
var
  i: Integer;
begin
  Result:='';
  for i:=Length(S) downto 1 do
    begin
      Result:=Result+Copy(S,i,1);
    end;
end;

Also der Code ist ja extremst grausam und unperformat.
Copy(S,i,1) = S[i] .
Und die Unmasse an String-Concationen ist sehr unperformant.

Delphi-Quellcode:

program Project5;

{$APPTYPE CONSOLE}

uses
  Windows, StrUtils;

function StringReverse_Copy(S: String): String;
var
  i: Integer;
begin
  Result := '';
  for i := Length(S) downto 1 do
    Result := Result + Copy(S, i, 1);
end;

function StringReverse_Char(S: String): String;
var
  i: Integer;
begin
  Result := '';
  for i := Length(S) downto 1 do
    Result := Result + S[i];
end;

function StringReverse_Direct(S: String): String;
var
  L, i: Integer;
begin
  L := Length(S);
  SetLength(Result, L);
  for i := L downto 1 do
    Result[i] := S[L - i + 2];
end;

var
  S, S2: String;
  C: Cardinal;
  i: Integer;

begin
  SetLength(S, 1234567);

  C := GetTickCount;
  for i := 0 to 9 do begin
    S2 := StringReverse_Copy(S);
    S2 := '';
  end;
  WriteLn(GetTickCount - C);

  C := GetTickCount;
  for i := 0 to 9 do begin
    S2 := StringReverse_Char(S);
    S2 := '';
  end;
  WriteLn(GetTickCount - C);

  C := GetTickCount;
  for i := 0 to 9 do begin
    S2 := StringReverse_Direct(S);
    S2 := '';
  end;
  WriteLn(GetTickCount - C);

  C := GetTickCount;
  for i := 0 to 9 do begin
    S2 := ReverseString(S);
    S2 := '';
  end;
  WriteLn(GetTickCount - C);

  ReadLn;
end.

Code:

2047
1296
110
47

Je länger der String, um so grausamer wird die nahezu expotentiell zur Zeichenanzahl steigene Rechenzeit.