表达式求值，注意：输入可能有\t！！！！没有考虑到，re到死
代码：
1 #include <iostream>
2 #include <stack>
3 #include <string.h>
4 #include <cstdio>
5 using namespace std;
6 const int maxx=1000;
7 int precede(char c)
8 {
9 switch(c)
10 {
11 case '+':
12 case '-':return 1;
13 case '*':
14 case '/':return 2;
15 default:return 0;
16 }
17 }
18
19 double getvalue(char c,double a,double b)
20 {
21 switch(c)
22 {
23 case '+':return b+a;
24 case '-':return b-a;
25 case '*':return b*a;
26 case '/':return b/a;
27 }
28 }
29
30 bool isnum(char c)
31 {
32 if(c>='0' && c<='9') return 1;
33 else return 0;
34 }
35
36 double strtonum(char s[],int &i)
37 {
38 double a=0.0;
39 a=(double)(s[i]-'0')*1.0;
40 i++;
41 while(isnum(s[i]))
42 {
43 a=a*10.0+(double)(s[i]-'0')*1.0;
44 i++;
45 }
46 return a;
47 }
48
49 void change(char c[],char s[])
50 {
51 int i,cnt=0,len=strlen(c);
52 s[0]='#';
53 for(i=0;i<len;)
54 {
55 if((c[i]=='+' || c[i]=='-') && s[cnt]=='#')
56 {
57 s[++cnt]='0';
58 s[++cnt]=c[i];
59 i++;
60 }
61 else if(c[i]=='(')
62 {
63 s[++cnt]=c[i];
64 i++;
65 while(c[i]==' ') i++;
66 if(c[i]=='+' || c[i]=='-')
67 {
68 s[++cnt]='0';
69 s[++cnt]=c[i];
70 i++;
71 }
72 }
73 else if(c[i]==' '||c[i]=='\t') i++;//坑爹啊！！！
74 else {s[++cnt]=c[i];i++;}
75 }
76 s[++cnt]='\0';
77 }
78
79 double result(char s[])
80 {
81 stack <char> ope;
82 stack <double> nd;
83 char c;
84 double a,b,ans;
85 int i,j=0,k,len=strlen(s);
86 for(i=1;i<len;)
87 {
88 if(isnum(s[i]))
89 {
90 a=strtonum(s,i);
91 nd.push(a);
92
93 }
94 else if(s[i]=='('){ ope.push('(');i++;}
95 else if(s[i]==')')
96 {
97 while(ope.top()!='(')
98 {
99 c=ope.top();ope.pop();
100 a=nd.top();nd.pop();
101 b=nd.top();nd.pop();
102 ans=getvalue(c,a,b);
103 nd.push(ans);
104 }
105
106 ope.pop();
107 i++;
108 }
109 else if(s[i]=='+' || s[i]=='-' || s[i]=='*' || s[i]=='/')
110 {
111 if(!ope.empty() && precede(s[i])<=precede(ope.top()))//若当前符号优先级比栈里边的小，则一直弹栈
112 {
113 c=ope.top();ope.pop();
114 a=nd.top();nd.pop();
115 b=nd.top();nd.pop();
116 ans=getvalue(c,a,b);
117 nd.push(ans);
118 }
119 else
120 {
121 ope.push(s[i]);
122 i++;
123 }
124 }
125 else i++;
126 }
127 while(!ope.empty())
128 {
129 c=ope.top();ope.pop();
130 a=nd.top();nd.pop();
131 b=nd.top();nd.pop();
132 ans=getvalue(c,a,b);
133 nd.push(ans);
134 }
135
136 return nd.top();
137 }
138
139
140 int main()
141 {
142 char s[maxx+100];
143 char c[maxx+100];
144 double ans;
145 // freopen("in.txt","r",stdin);
146 while(gets(c))
147 {
148 change(c,s);
149 ans=result(s);
150 printf("%.4lf\n",ans);
151
152 }
153 return 0;
154 }