前缀表达式的计算

//nyoj 128前缀表达式#include <iostream>#include<stdio.h>

#include <sstream>#include <iomanip>#include <cstring>#include <stack>using namespace std;char Compare(char c,char d){switch (c){case '+':case '-': if(d=='+'||d=='-') return '>'; else return '<';case '*':case '/':return '>';default:break;}}bool IsChar(char c){if(c=='+'||c=='-'||c=='*'||c=='/')return true;return false;}void Change(char mid[],char post[]){int i=0,j=-1,l=strlen(mid);stack<char> s;int start,end,flag=0,k,p;char temp[20];char ch;while (i<l){if(IsChar(mid[i])||mid[i]==' ')  {if(flag)  {p=j;j=0; for(k=end;k>=(end-p);k--) mid[k]=temp[j++]; j=-1;flag=0; memset(temp,0,sizeof(temp));} ++i;continue;}if((mid[i]>='0'&&mid[i]<='9')||(mid[i]=='.')){end=i;flag=1;temp[++j]=mid[i];i++;}}if(flag)  {p=j;j=0; for(k=end;k>=(end-p);k--) mid[k]=temp[j++]; j=-1;flag=0; memset(temp,0,sizeof(temp));}//处理完小数点的问题，下面开始转后面表达式i=0;while(i<l) {s.push(mid[i]);++i;}i=0;while(i<l) {ch=s.top();s.pop(); post[i++]=ch;}}double To(char a[]){stringstream oss;oss<<a;double result;oss>>result;return result;}double Operator(double a,char  b,double c){switch (b){case '+': return a+c;case '-': return a-c;case '*': return a*c;case '/': return a/c;}}double run(char post[]){stack<double> s;double result,x,y;char temp[20];memset(temp,0,sizeof(temp));int i=0,l=strlen(post),j=-1,flag=0;while (i<l){if((post[i]>='0'&&post[i]<='9')||(post[i]=='.')){temp[++j]=post[i];flag=1;++i;}else{if(flag==1){result=To(temp); s.push(result); j=-1;flag=0; memset(temp,0,sizeof(temp));}if(post[i]==' ') { ++i;continue;}else if(IsChar(post[i])){x=s.top(); s.pop();y=s.top(); s.pop();s.push(Operator(x,post[i],y));++i;}}}return s.top();}char mid[1008],post[1008];int main(){while (gets(mid)){Change(mid,post);cout<<setiosflags(ios::fixed)<<setprecision(2)<<run(post)<<endl;memset(mid,0,sizeof(mid));memset(post,0,sizeof(post));}return 0;}