表达式求值

 

///////////////////////////////////////////////////////////////////////////

// 作者: 游侠UFO

// 时间: 2008年月日

// 版本: 1.1

// 备注: 需包含头文件<string>、<stack>；表达式(strExp)需以'='结束。

///////////////////////////////////////////////////////////////////////////

 

long Evaluate(const string &strExp)

{

    stack<size_t> stkiOptr;

    stack<long> stklOpnd;

    long lAns=0;

    size_t iOptr=6, Index=0;

    string strOpnd, strOptrTable="+-*/()=";

    int iOperator[7][7]={

       {2, 2, 0, 0, 0, 2, 2},

       {2, 2, 0, 0, 0, 2, 2},

       {2, 2, 2, 2, 0, 2, 2},

       {2, 2, 2, 2, 0, 2, 2},

       {0, 0, 0, 0, 0, 1,-1},

       {2, 2, 2, 2,-1, 2, 2},

       {0, 0, 0, 0, 0,-1, 1} };

 

    stkiOptr.push(6);

    stklOpnd.push(0);

    while (strExp[Index]!='=' || iOptr!=6)

    {

       if (48<=strExp[Index] && strExp[Index]<=57) strOpnd.push_back(strExp[Index++]);

       else

       {

           size_t iTemp, iOptrTemp;

 

           if ((iTemp=strOptrTable.find(strExp[Index]))!=string::npos)

           {

              if (!strOpnd.empty())

              {

                  stklOpnd.push(atoi(strOpnd.data()));

                  strOpnd="";

              }

              iOptrTemp=iTemp;

              iOptr=stkiOptr.top();

              switch (iOperator[iOptr][iOptrTemp])

              {

              case 0:

                  stkiOptr.push(iOptrTemp);

                  Index++;

                  break;

              case 1:

                  iOptr=stkiOptr.top();

                  stkiOptr.pop();

                  Index++;

                  break;

              case 2:

                  long a, b;

 

                  iOptrTemp=stkiOptr.top();

                  stkiOptr.pop();

                  b=stklOpnd.top();

                  stklOpnd.pop();

                  a=stklOpnd.top();

                  stklOpnd.pop();

                  switch (iOptrTemp)

                  {

                  case 0:

                     stklOpnd.push(a+b);

                     break;

                  case 1:

                     stklOpnd.push(a-b);

                     break;

                  case 2:

                     stklOpnd.push(a*b);

                     break;

                  case 3:

                     stklOpnd.push(a/b);

                     break;

                  }

                  break;

              }

           }

       }

       iOptr=stkiOptr.top();

    }

    lAns=stklOpnd.top();

    stklOpnd.pop();

    return lAns;

}