运用栈实现表达式求值（+，-，*，/运算）

 1 #include <iostream>
 2 #include <string>
 3 #include <cassert>
 4 #include <iomanip>
 5 using namespace std;
 6 template<typename Type> class Stack {
 7 private:
 8     Type *elements;
 9     int max_size, top_index;
10 public:
11     Stack(int length_input) {
12         elements = new Type[length_input];
13         max_size = length_input;
14         top_index = -1;
15     }
16     ~Stack() {
17         delete[] elements;
18     }
19     bool push(const Type &element) {
20         if (top_index >= max_size - 1) {
21             return false;
22         }
23         top_index++;
24         elements[top_index] = element;
25         return true;
26     }
27     bool pop() {
28         if (top_index < 0) {
29             return false;
30         }
31         top_index--;
32         return true;
33     }
34     Type top() {
35         assert(top_index >= 0);
36         return elements[top_index];
37     }
38     bool empty() {
39         return top_index < 0;
40     }
41 };
42 
43 static char pre[128] = { 0 };
44 
45 void setpre()
46 {
47     pre['+'] = 1;
48     pre['-'] = 1;
49     pre['*'] = 2;
50     pre['/'] = 2;
51 }
52 
53 bool precede(char op1, char op2) {
54     return pre[op1] > pre[op2] ? true : false;
55 }
56 double operate(char theta, double a, double b) {
57     switch (theta) {
58         case '-': return b - a; break;
59         case '+': return a + b; break;
60         case '*': return a * b; break;
61         case '/': return (b * 1.0) / a; break;
62     }
63 }
64 void calc(Stack<double> &numbers, Stack<char> &operators) {
65     double a = numbers.top();
66     numbers.pop();
67     double b = numbers.top();
68     numbers.pop();
69     numbers.push(operate(operators.top(), a, b));
70     operators.pop();
71 }
72 int main() {
73     int n;
74     setpre();
75     cin >> n;
76     Stack<double> numbers(n);
77     Stack<char> operators(n);
78     string buffer;
79     cin >> buffer;
80     int i = 0;
81     while (i < n) {
82         if (isdigit(buffer[i])) {
83             numbers.push(buffer[i] - '0');
84             i++;
85         } else {
86             if (operators.empty() || precede(buffer[i], operators.top())) {
87                 operators.push(buffer[i]);
88                 i++;
89             } else {
90                 calc(numbers, operators);
91             }
92         }
93     }
94     while (!operators.empty()) {
95         calc(numbers, operators);
96     }
97     cout << fixed << setprecision(3) << numbers.top() << endl;
98     return 0;
99 }