lw549說 取得字符串中指定子字符串出現(xiàn)第n次的位置,效率不高,勉強可用 。感上興趣，于是試上一試。
程序附在最后，這里是一些說明文字：
1、為快速寫好，沒有使用應當使用的控制臺方式，而是使用了GUI方式；
2、測試的樣例是查找包含有四處子串的字符串，四次分別查四個位置。這個在Button1Click方法中完成，它調用Tests來進行具體測試，以被測函數(shù)、第幾次出現(xiàn)、循環(huán)次數(shù)為參數(shù)；
3、Tests依次在一個循環(huán)中重復調用每個具體的函數(shù)，同時為了公平起見（也許前面的函數(shù)為后面的鋪了一些路——內(nèi)存、高速緩沖），這樣的測試進行TEST_COUNT次，最后輸出每次的平均時間；
4、經(jīng)過前期的測試，lw549 的代碼的確效率不高，所以單獨給它小一些的循環(huán)次數(shù)（一千次），以免造成程序假死現(xiàn)象；其它的為十萬次；
5、其它三個函數(shù)的思想為：
PosN_Pos: 使用Pos函數(shù)及Copy函數(shù)；
PosN_PosEx: 使用Delphi 7中增加的PosEx函數(shù)；
PosN_StrPos: 使用StrPos函數(shù)。

程序輸出：

可以看出，測試的結果（效率）是：  PosN_PosEx > PosN_StrPos > PosN_Pos >> GetNSubStringPos 。
我本來期望的是 PosN_StrPos 最厲害，但結果不是。估計是 PosEx 優(yōu)化得比較厲害。

附代碼：

Unit1.pas:

unit Unit1;interfaceuses  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,  Dialogs, StdCtrls, DB, DBTables;type  TPosNFunc = function (N: Integer; const SubString,AString: String): Integer;type  TForm1 = class(TForm)    Memo1: TMemo;    Button1: TButton;    PRocedure Button1Click(Sender: TObject);    procedure FormCreate(Sender: TObject);  private    procedure Tests(const funcs: array of TPosNFunc;      const funcnames: array of string; iStr: integer; loopcount: integer);  public  end;var  Form1: TForm1;implementationuses  StrUtils;{$R *.dfm}function GetNSubStringPos(N: Integer; SubString,AString: String): Integer;//返回第n個SubString在AString中出現(xiàn)的位置//如果沒找到,返回-1var  FindCount: Integer;  Pos: Integer;begin  Result := -1;  Pos := 0;  for FindCount := 1 to N do begin    Inc(Pos);    while MidStr(AString, Pos, Length(SubString)) <> SubString do begin      if Length(AString) < Length(SubString) + Pos then        Exit;//未找到      Inc(Pos);    end;  end;  Result := Pos;end;function PosN_Pos(N: Integer; SubString, AString: String): Integer;var  p: integer;  nSub: integer;  nSrc: integer;begin  nSub := Length( SubString );  nSrc := Length( AString );  result := -nSub;  while N>0 do  begin    p := Pos(SubString, AString);    if p=0 then      break;    Dec( N );    Inc( result, p+nSub );    AString := Copy( AString, p+nSub+1, nSrc-nSub-p-1 );    Dec( nSrc, nSub+p );  end;  if N>0 then    result := -1;end;function PosN_PosEx(N: Integer; SubString,AString: String): Integer;var  p: integer;  nSub: integer;begin  nSub := Length( SubString );  result := 0;  p := 0;  while N>0 do  begin    p := PosEx( SubString, AString, p+1 );    if p=0 then      break;    Dec( N );    result := p;    Inc( p, nSub );  end;  if N>0 then    result := -1;end;function PosN_StrPos(N: Integer; SubString, AString: String): Integer;var  pSub, pSrc, p: Pchar;  nSub: integer;begin  nSub := Length( SubString );  pSub := PChar(SubString);  pSrc := PChar(AString);  p := pSrc;  while (N>0) do  begin    p := StrPos( p, pSub );    if (p=nil) then      break;    Inc( p, nSub );    Dec( N );  end;  if (N=0) and (p<>nil) then    result := p - pSrc  else    result := 0;end;const  STR = 'function GetNSubStringPos(N: Integer; SubString,AString: String): Integer;';  SUBSTR = 'String';  TEST_COUNT = 5;procedure TForm1.Tests( const funcs: array of TPosNFunc;                        const funcnames: array of string;                        iStr: integer;                        loopcount: integer );var  i, j, k: Integer;  tm: LongWord;  func: TPosNFunc;  count: integer;  retv: array of integer;  results: array of Longword;begin  count := Length(funcs);  assert( count=Length(funcnames) );  Memo1.Lines.Add( Format('substr index: %d; LOOP COUNT = %d',                          [iStr, loopCount]) );  SetLength( retv, count );  SetLength( results, count );  for j:=0 to count-1 do    results[j] := 0;  for k:=1 to TEST_COUNT do  begin    for j:=0 to count-1 do    begin      func := funcs[j];      tm := GetTickCount;      for i:=1 to loopcount do        retv[j] := func( iStr, SUBSTR, str );      Inc( results[j], GetTickCount - tm );    end;  end;  for j:=0 to count-1 do  begin    Memo1.Lines.Add( Format( '%s: return %d; Timing: %n ms',                            [funcnames[j], retv[j],                            results[j]*1.0/TEST_COUNT ] ) );  end;end;procedure TForm1.Button1Click(Sender: TObject);var  i: integer;begin  for i:=1 to 4 do  begin    Memo1.Lines.Add( Format( '%d:', [i]) );    Tests( [@GetNSubStringPos], ['GetNSubStringPos'], i, 1000 );    Tests( [@PosN_Pos, @PosN_PosEx, @PosN_StrPos],           ['PosN_Pos', 'PosN_PosEx', 'PosN_StrPos'],           i, 100000 );  end;end;procedure TForm1.FormCreate(Sender: TObject);begin  Memo1.Clear;  Memo1.Lines.Add( Format( 'Search "%s" for "%s"', [STR, SUBSTR] ) );end;end.

Unit1.dfm:

object Form1: TForm1  Left = 243  Top = 164  Width = 578  Height = 516  AlphaBlendValue = 192  Caption = 'Form1'  Color = clBtnFace  Font.Charset = DEFAULT_CHARSET  Font.Color = clWindowText  Font.Height = -11  Font.Name = 'MS Sans Serif'  Font.Style = []  OldCreateOrder = False  Position = poDefaultPosOnly  OnCreate = FormCreate  DesignSize = (    570    489)  PixelsPerInch = 96  TextHeight = 13  object Memo1: TMemo    Left = 3    Top = 32    Width = 565    Height = 457    Anchors = [akLeft, akTop, akRight, akBottom]    Lines.Strings = (      'Memo1')    ScrollBars = ssVertical    TabOrder = 0  end  object Button1: TButton    Left = 3    Top = 6    Width = 75    Height = 25    Caption = 'Button1'    TabOrder = 1    OnClick = Button1Click  endend