Delphi 计算字符串表达式

function Exp(SExpression:string):Double;
  function GetValue(SEXP:string):Currency;
  Begin
    With DM Do
    Begin
      If SEXP='W' then Result:=cdsOrderTempletWindowsWidth.AsCurrency //  针对特定的变量代替字符取相应的字段值计算
      Else If SEXP<>'' Then Result:=StrToCurrDef(SEXP,0);
    end;
  end;
const
  EX='+-*/()';
Var
  SExp,SVar,PX:String;
  P,Len,LCount:Integer;
  Value:Double;
Begin
  SExp:=StringReplace(Trim(SExpression),#32,'',[rfReplaceAll]);
  Len:=Length(SExp);
  Result:=0;
  P:=1;
  SVar:='';
  PX:='';
  LCount:=0;
  With DM Do
  Begin
    while P<=Len Do
    Begin
      If Pos(SExp[P],EX)>0 then
      Begin
        Value:=0;