后缀表达式

  1 // 后缀表达式的计算
  2 #include <iostream>
  3 #include <sstream>
  4 #include <vector>
  5 #include <stack>
  6 #include <map>
  7 #include <string>
  8 using namespace std;
  9 
 10 void get_postfix(vector<string>& postf)
 11 {
 12     postf.clear();
 13     string line;
 14     getline(cin, line);
 15     istringstream sin(line);
 16     string tmp;
 17     while (sin >> tmp)
 18     {
 19         postf.push_back(tmp);
 20     }
 21 }
 22 
 23 void init_op(map<string, int>& ops)
 24 {
 25     ops.clear();
 26     ops["+"] = 100;
 27     ops["-"] = 100;
 28     ops["*"] = 200;
 29     ops["/"] = 200;
 30     ops["("] = 1000;
 31     ops[")"] = 0;
 32 }
 33 
 34 bool is_operator(const string& hs, const map<string, int>& ops)
 35 {
 36     map<string, int>::const_iterator cit = ops.find(hs); 37     if (cit != ops.end())
 38     {
 39         return true;
 40     }
 41     else
 42     {
 43         return false;
 44     }
 45 }
 46 
 47 double cal_post(const vector<string>& postf, const map<string, int>& ops)
 48 {
 49     stack<double> or_st;
 50     double operand = 0.0, a = 0.0, b = 0.0, c = 0.0;
 51     for (vector<string>::size_type i = 0; i != postf.size(); ++i)
 52     {
 53         if (!is_operator(postf[i], ops))
 54         {
 55             operand = static_cast<double>(atof(postf[i].c_str()));
 56             or_st.push(operand);
 57         }
 58         else
 59         {
 60             switch (postf[i][0])
 61             {
 62             case ‘+‘:
 63                 b = or_st.top();
 64                 or_st.pop();
 65                 a = or_st.top();
 66                 or_st.pop();
 67                 c = a + b;
 68                 or_st.push(c);
 69                 break;
 70             case ‘-‘:
 71                 b = or_st.top();
 72                 or_st.pop();
 73                 a = or_st.top();
 74                 or_st.pop();
 75                 c = a - b;
 76                 or_st.push(c);
 77                 break;
 78             case ‘*‘:
 79                 b = or_st.top();
 80                 or_st.pop();
 81                 a = or_st.top();
 82                 or_st.pop();
 83                 c = a * b;
 84                 or_st.push(c);
 85                 break;
 86             case ‘/‘:
 87                 b = or_st.top();
 88                 or_st.pop();
 89                 a = or_st.top();
 90                 or_st.pop();
 91                 c = a / b;
 92                 or_st.push(c);
 93                 break;
 94             default:
 95                 break;
 96             }
 97         }
 98     }
 99     if (or_st.size() == 1)
100     {
101         return or_st.top();
102     }
103     else
104     {
105         return -10000000000000.0;
106     }
107 }
108 
109 int main()
110 {
111     map<string, int> ops;
112     init_op(ops);
113     vector<string> postf;
114     
115     while (1)
116     {
117         get_postfix(postf);
118         double ret = cal_post(postf, ops);
119         cout << ret << endl << endl;
120     }
121     
122     system("PAUSE");
123     return 0;
124 }