lw549说 取得字符串中指定子字符串出现第n次的位置,效率不高,勉强可用 。感上兴趣，于是试上一试。
程序附在最后，这里是一些说明文字：
1、为快速写好，没有使用应当使用的控制台方式，而是使用了GUI方式；
2、测试的样例是查找包含有四处子串的字符串，四次分别查四个位置。这个在Button1Click方法中完成，它调用Tests来进行具体测试，以被测函数、第几次出现、循环次数为参数；
3、Tests依次在一个循环中重复调用每个具体的函数，同时为了公平起见（也许前面的函数为后面的铺了一些路——内存、高速缓冲），这样的测试进行TEST_COUNT次，最后输出每次的平均时间；
4、经过前期的测试，lw549 的代码的确效率不高，所以单独给它小一些的循环次数（一千次），以免造成程序假死现象；其它的为十万次；
5、其它三个函数的思想为：
PosN_Pos: 使用Pos函数及Copy函数；
PosN_PosEx: 使用Delphi 7中增加的PosEx函数；
PosN_StrPos: 使用StrPos函数。

程序输出：

可以看出，测试的结果（效率）是：  PosN_PosEx > PosN_StrPos > PosN_Pos >> GetNSubStringPos 。
我本来期望的是 PosN_StrPos 最厉害，但结果不是。估计是 PosEx 优化得比较厉害。

附代码：

Unit1.pas:

unit Unit1;

interface

uses

  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;

implementation

uses

  StrUtils;

{$R *.dfm}

function GetNSubStringPos(N: Integer; SubString,AString: String): Integer;

//返回第n个SubString在AString中出现的位置

//如果没找到,返回-1

var

  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

  end

end