一个计算器的代码，欢迎大家点评

例如：
   1. CalcExpr('2*5+1')='11'
   2. 带条件
       CalcExpr('2>1&4<=5 : 2*5')='10'
       CalcExpr('6<2 : 3')='0'
   3. 带函数
       CalcExpr('max(1,2,3,6,4+7,7)')='11'

用法：将untCalc.pas 加入到你的工程里面，然后调用CalcExpr即可。

这里是源代码：

unit untJCalc;

interface

uses
   classes,sysutils;

type
   TJStack=class
      private
         Lines:TStrings;
      public
         constructor Create;
         destructor Destroy;
         procedure init;
         procedure push(s:string);
         function GetTop:String;
         function Pop:String;
      end;
   TJExpr=class
      private
         Expr:String;
         Position:Integer;
         Min,max:Integer;
         Eof:Boolean;
      public
         constructor Create(pExpr:String);
         function read:String;
         procedure GoFirst;
      end;

function CalcExpr(sExpr:String):String;
function CalcExprItem(sOptr,sA,sB:String):String;
function OptrIndex(w:string):Integer;
function GetParamCount(pFunc:String):Integer;
function ExecFunc(pFunc:String;pParam:Array  of string;pParamCount:Integer):string;

implementation

constructor TJStack.Create;
begin
   inherited Create;
   lines:=TStringList.create;
end;

procedure TJStack.init;
begin
   lines.free;
end;

destructor TJStack.Destroy;
begin
   lines.free;
   inherited Destroy;
end;

procedure TJStack.push(s:string);
begin
   lines.add(s);
end;

function TJStack.GetTop:String;
begin
   if Lines.count>0 then
      Result:=lines[lines.count-1]
      else
      Result:='';
end;

function TJStack.Pop:String;
begin
   if Lines.Count>0 then
   begin
      Result:=GetTop;
      lines.delete(lines.count-1);
   end
   else
      Result:='';
end;

//////////////////////TJExpr////////////////

constructor TJExpr.Create(pExpr:String);
begin
   Expr:=lowercase(pExpr)+'#';
   Min:=1;
   Max:=length(Expr);
   Position:=1;
   Eof:=false;
end;

function TJExpr.read:String;
   function SameType(s1,s2:string):boolean;
   var
      c1,c2:string;
   begin
      c1:='';c2:='';
      if length(s1)>0 then c1:=s1[length(s1)];
      if length(s2)>0 then c2:=s2[Length(s2)];
      if ((pos(c1,'0123456789.')>0) and (pos(c2,'0123456789.')>0))
         then
         begin
            result:=true;
         end
         else
         begin
            Result:=false;
         end;
      if (c1='-')and(c2='-') then Result:=false;
      if s1+s2='>=' then Result:=true;
      if s1+s2='<=' then Result:=true;
      if s1+s2='<>' then Result:=true;
      if pos(s1+s2,'max(')>0 then Result:=true;
      if pos('-',s1+s2)>1 then Result:=false;
      if (s1='')or(s2='') then result:=true;
   end;
begin
   if Position<=Max then
   begin
      Result:=trim(Expr[Position]);