如何计算字符型表达式

Question

如何计算字符型表达式如：12+3*5/10的值，返回数值型结果13。5如:
var 
  aaa:string;
begin
  aaa:='(12+3*6)/3';
  如何得到aaa的值返回10为数值型.
因为表达式可能为aaa:='25+6*9'
其他语言如:access中有相应的函数;exl

Answer 1

bbb:integer;
如何使bbb:=aaa
即得到,结果10;

Answer 2

我的方法是解析字符串，然后用两个链表分别存储数字和操作符，之后循环存储操作符的链表，用位置比对关系得出整个串的运算结果，我刚才做了个例子，因为时间短，我没有把扩号加进来（因为运算量会加很多），只是简单的实现了加减乘除，现在贴出来给楼主看看，如果楼主有兴趣，可以自行考虑加时间的情况以及纠错处理（就是字符串不规则，比如两个运算符号相连或者除数为0等等）unit Unit1;interfaceuses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;type
  TForm1 = class(TForm)
    Button1: TButton;
    Edit1: TEdit;
    Edit2: TEdit;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
    valuelist:tstringlist;
    fhlist:tstringlist;
    function mathways(mathstr:string):string;
    procedure putvaluetolist(s1:string);
    function listisnull:boolean;
  end;var
  Form1: TForm1;implementation{$R *.dfm}procedure TForm1.Button1Click(Sender: TObject);
begin
    self.Edit2.Text:=self.mathways(self.Edit1.Text);
end;
//-----------------------------------------------------------------------------
function TForm1.mathways(mathstr:string):string;
var
   s1:string;
   i,icount:integer;
   str,str1,str2:string;
begin
    valuelist:=tstringlist.Create();
    fhlist:=tstringlist.Create();
    self.putvaluetolist(mathstr);
    while not self.listisnull do
    begin
        for i:=0 to fhlist.Count-1 do
        begin
            if (fhlist.Strings[i]='*')or(fhlist.Strings[i]='/') then
            begin
                str1:=valuelist.Strings[i];
                str2:=valuelist.Strings[i+1];
                if fhlist.Strings[i]='*' then
                begin
                    str:=floattostr(strtofloat(str1)*strtofloat(str2));
                    valuelist.Strings[i]:=str;
                    valuelist.Delete(i+1);
                    fhlist.Delete(i);
                end
                else if fhlist.Strings[i]='/' then
                begin
                    str:=floattostr(strtofloat(str1)/strtofloat(str2));
                    valuelist.Strings[i]:=str;
                    valuelist.Delete(i+1);
                    fhlist.Delete(i);
                end;
                break;
            end;        end;
    end;
    while  fhlist.Count>0 do
    begin
        for i:=0 to fhlist.Count-1 do
        begin            str1:=valuelist.Strings[i];
            str2:=valuelist.Strings[i+1];
            if fhlist.Strings[i]='+' then
            begin
                str:=floattostr(strtofloat(str1)+strtofloat(str2));
                valuelist.Strings[i]:=str;
                valuelist.Delete(i+1);
                fhlist.Delete(i);
            end
            else if fhlist.Strings[i]='-' then
            begin
                str:=floattostr(strtofloat(str1)-strtofloat(str2));
                valuelist.Strings[i]:=str;
                valuelist.Delete(i+1);
                fhlist.Delete(i);
            end;
            break
        end;
    end;
    s1:=valuelist.Strings[0];
    valuelist.Free;
    fhlist.Free;
    result:=s1;
end;
procedure TForm1.putvaluetolist(s1:string);
var
    i:integer;
    icount:integer;
    s2:string;
begin
    icount:=0;
    for i:=1 to length(s1) do
    begin
        if (s1[i]='*') or(s1[i]='/') or(s1[i]='+') or(s1[i]='-') then
        begin
            valuelist.Add(copy(s1,icount+1,i-icount-1));
            fhlist.Add(copy(s1,i,1));
            icount:=i;
        end;
        if i=length(s1) then
        begin
            valuelist.Add(copy(s1,icount+1,i-icount));
        end;
    end;
end;
function tform1.listisnull:boolean;
var
    flag:boolean;
    i:integer;
begin
    flag:=true;
    for i:=0 to fhlist.Count-1 do
    begin
        if (fhlist.Strings[i]='*') or(fhlist.Strings[i]='/') then
        begin
            flag:=false;
            break;
        end;
    end;
    result:=flag;
end;
end.