求前缀表达式的值

  1 #include <stdlib.h>
  2 #include <stdio.h>
  3 #include <string.h>
  4 #include <cstdio>
  5 #include <stack>
  6 #include <cmath>
  7 
  8 using namespace std;
  9 stack<int> instack;
 10 stack<double> mystack;
 11 
 12 void stack(stack<int> mystack)
 13 {
 14   if(mystack.size()>=0)
 15     {
 16       int n=mystack.top();
 17       printf("%d",n);
 18       mystack.pop();
 19     }
 20 }
 21 
 22 int main()
 23 {
 24  
 25   int i=0;
 26   int dotpos=0;
 27   char str[40];
 28   while(gets(str))
 29     {
 30       for(i=strlen(str)-1;i>=0;i--)
 31     {
 32       if(str[i]=='.')
 33         {
 34           dotpos=instack.size();
 35           continue;
 36         }
 37 
 38       if(str[i]=='+'||str[i]=='-'||str[i]=='*'||str[i]=='/')
 39         {
 40           if(str[i]=='-'&& instack.size()!=0)
 41         {
 42           int icount=0;
 43           double itemp=0;
 44           int len=instack.size();
 45           while(instack.size()>0)
 46             {
 47               double inttemp=instack.top();
 48               instack.pop();
 49               itemp=inttemp*pow(10,len-1-icount)+itemp;
 50               icount++;
 51             }
 52           itemp=itemp*-1;
 53           if(dotpos!=0)
 54             {
 55             mystack.push(itemp / pow(10,dotpos));
 56             dotpos=0;
 57             }
 58           else 
 59             mystack.push(itemp);
 60           continue;
 61 
 62         }
 63 
 64            if(str[i]=='+'&& instack.size()!=0)
 65         {
 66           int icount=0;
 67           double itemp=0;
 68           int len=instack.size();
 69           while(instack.size()>0)
 70             {
 71               double inttemp=instack.top();
 72               instack.pop();
 73               itemp=inttemp*pow(10,len-1-icount)+itemp;
 74               icount++;
 75             }
 76           if(dotpos!=0)
 77             {
 78             mystack.push(itemp / pow(10,dotpos));
 79             dotpos=0;
 80             }
 81           else 
 82             mystack.push(itemp);
 83           continue;
 84 
 85         }
 86            if(mystack.size()<2)
 87          {
 88          printf("ERROR");
 89          return 0;
 90              }
 91            double a=mystack.top();
 92            mystack.pop();
 93            double b=mystack.top();
 94            mystack.pop();
 95            double r=0;
 96            switch(str[i])
 97          {
 98          case '+': r=a+b; break;
 99          case '-':r=a-b; break;
100          case '*':r=a*b;break;
101          case '/':
102               if(b==0)
103                 {
104                   printf("ERROR");
105                   return 0;
106                 }
107               else r= a / b ;
108               break;
109          }
110 
111               mystack.push(r);
112               continue;
113         }
114       if(str[i]==' ')
115         {
116                  int icount=0;
117           double itemp=0;
118           int len=instack.size();
119           while(instack.size()>0)
120             {
121               double inttemp=instack.top();
122               instack.pop();
123               itemp=inttemp*pow(10,len-1-icount)+itemp;
124               icount++;
125             }
126           if(len!=0)
127             {
128           if(dotpos!=0)
129             {
130             mystack.push(itemp / pow(10,dotpos));
131             dotpos=0;
132             }
133           else 
134             mystack.push(itemp);
135             }
136           continue;
137         }
138          
139       if(str[i]>='0'&&str[i]<='9')
140         {
141           instack.push(str[i]-'0');
142           continue;
143 
144         }
145     }
146       printf("%.1lf
",mystack.top());
147     }
148 
149 
150   return 0;
151 }