字符串表达式求值（支持多种类型运算符）

 1 /* 字符串表达式求值
 2  * @param input: 输入的字符串
 3  * @param output: 表达式的值，若含有关系运算符则为1或者0
 4  * return 计算过程是否正常
 5  */
 6 bool ExpValue(string input,int& output)
 7 {
 8     stack<int> operand_stack;
 9     stack<string> operator_stack;
10 
11     char prev = 0; // 上一个属于运算符的字符
12     for (int i = 0; i < input.size(); i++)
13     {
14         char c = input[i];
15         // prev是否是一个完整运算符
16         if (!isOperator(c) && prev)
17         {
18             string new_op = string("").append(1, prev);
19             addNewOperator(new_op, operand_stack, operator_stack);
20             prev = 0;
21         }
22 
23         // 数字
24         if (isdigit(c))
25         {
26             int val_c = c - '0';
27             if (i > 0 && isdigit(input[i - 1]))
28             {
29                 int top_num = operand_stack.top();
30                 top_num = top_num * 10 + val_c;
31                 operand_stack.pop();
32                 operand_stack.push(top_num);
33             }
34             else
35                 operand_stack.push(val_c);
36         }
37         // 运算符字符
38         else if (isOperator(c))
39         {
40             // 处理两字符运算符
41             if (prev)
42             {
43                 string new_op = string("").append(1, prev).append(1, c);
44                 addNewOperator(new_op, operand_stack, operator_stack);
45                 prev = 0;
46             }
47             else
48                 prev = c;
49         }
50         else if (c == '(')
51             operator_stack.push("(");
52         else if (c == ')')
53         {
54             // 处理括号内的运算符
55             while (operator_stack.top()!="(")
56             {
57                 int num1 = operand_stack.top();
58                 operand_stack.pop();
59                 int num2 = operand_stack.top();
60                 operand_stack.pop();
61                 string op = operator_stack.top();
62                 operator_stack.pop();
63 
64                 int val = Calculate(num2, num1, op);
65                 operand_stack.push(val);
66             }
67             operator_stack.pop(); // 弹出"("
68         }
69     }
70     assert(operand_stack.size() == operator_stack.size() + 1);
71     // 弹出所有运算符
72     while(!operator_stack.empty())
73     {
74         int num2 = operand_stack.top();
75         operand_stack.pop();
76         int num1 = operand_stack.top();
77         operand_stack.pop();
78         string op = operator_stack.top();
79         operator_stack.pop();
80 
81         int val = Calculate(num1, num2, op);
82         operand_stack.push(val);
83     }
84 
85     if (operand_stack.size() == 1) {
86         output = operand_stack.top();
87         return true;
88     }
89     return false;
90 }

int main()
{
    string s0 = "10-1*10+3%2";
    string s1 = "100 + (3-33)*2";
    string s2 = "20+1 >= 20 && 20+1 < 20";
    string s3 = "10>20 || 10/1>=5";
    int ret = -1;
    if (ExpValue(s0, ret))
        cout << s0 << "的值: " << ret << endl;

    if (ExpValue(s1, ret))
        cout << s1 << "的值: " << ret << endl;

    if (ExpValue(s2, ret))
        cout << s2 << "的值: " << ret << endl;

    if (ExpValue(s3, ret))
        cout << s3 << "的值: " << ret << endl;
    return 0;
}