简单的表达式计算 c++

 1 #include<iostream>
 2 #include<stack>
 3 #include<string>
 4 #include<vector>
 5 #include<map>
 6 #include<algorithm>
 7 using namespace std;
 8 
 9 map<const char, int> priority;//用map来储存运算符的优先级
10 
11 int compare(char a, char b){
12     int cmp = 0;
13     if(b==‘)‘ && a==‘(‘) cmp = 0;
14     else if(a==‘(‘) cmp = -1;
15     else if(priority[a] < priority[b]) cmp = -1;
16     else if(priority[a] >= priority[b]) cmp = 1;
17     return cmp;
18 }
19 
20 int cal(int a, int b, char op){
21     int ans;
22     if(op==‘+‘) ans = a+b;
23     else if(op==‘-‘) ans = a-b;
24     else if(op==‘*‘) ans = a*b;
25     else if(op==‘/‘) ans = a/b;
26     return ans;
27 }
28 
29 int calculator(){
30     //根据优先级的关系，可以把他们的优先级定义为下面的形式
31     //个别运算符的优先级是与位置前后有关系的，需要特俗处理
32     priority[‘#‘] = -1;
33     priority[‘)‘] = 0;
34     priority[‘+‘] = 1;
35     priority[‘-‘] = 1;
36     priority[‘*‘] = 2;
37     priority[‘/‘] = 2;
38     priority[‘(‘] = 3;
39     cout<<"please input valid expression, enter to terminate..."<<endl;
40     char ch = getchar();
41     stack<char> op;
42     stack<int> nums;
43     op.push(‘#‘);
44     nums.push(0);
45     bool flag = true;
46     while(ch!=‘#‘ || op.top()!=‘#‘){
47         if(ch<=‘9‘ && ch>=‘0‘){
48             int number = 0;
49             while(ch>=‘0‘ && ch<=‘9‘){//连续出现的数字看做一个整体
50                 number = number*10 + (ch-‘0‘);
51                 ch = getchar();
52             }
53         //    cout<<"number: "<<number<<endl;
54         //    cout<<"op: "<<ch<<endl;
55             nums.push(number);
56             flag = true;
57         }else{//比较栈顶运算符和新输出运算符的优先级
58             int cmp = compare(op.top(), ch);
59             //cout<<"compare("<<op.top()<<","<<ch<<") = "<<cmp<<endl;
60             if(cmp==-1){//顶部优先级低时，把新的运算符压栈
61                 op.push(ch);
62                 flag = false;
63                 ch = getchar();
64             }else if(cmp==0){//即栈顶和新的运算符是‘(‘和‘)‘，需要把‘(‘弹出
65                 op.pop();
66                 ch = getchar();
67             }else if(cmp==1){//栈顶运算符优先级高的时候，就要进行运算
68                 int num1, num2, tempans;
69                 char tempop;//一定要注意num的顺序，否则会导致错误的运算结果
70                 num2 = nums.top();
71                 nums.pop();
72                 num1 = nums.top();
73                 nums.pop();
74                 tempop = op.top();
75                 op.pop();
76                 tempans = cal(num1, num2, tempop);
77                 //cout<<"tempans: "<<tempans<<endl;
78                 nums.push(tempans);
79             }
80             if(ch==‘-‘ && !flag) nums.push(0);
81             
82         }
83     }
84     cin.get();
85     return nums.top();
86 }
87 int main(){
88     int i = 10;
89     while(i--){
90         cout<<calculator()<<endl;
91     }
92 return 0;}