02-线性结构3. 求前缀表达式的值（25）

+ + 2 * 3 - 7 4 / 8 4

13.0

/ -25 + * - 2 3 4 / 8 4

12.5

/ 5 + * - 2 3 4 / 8 2

ERROR

+10.23

10.2

#include <iostream>
#include <malloc.h>
#include <string.h>
#include <stdio.h>
using namespace std;

int main()
{
char s[200];
int len,i,k=0;
double a[200];
gets(s);
len=strlen(s);
for(i=len-1;i>=0;i--)
{
//cout<<i<<"="<<s[i]<<endl;
if(s[i]==' ')
continue;
if(s[i]=='+'||s[i]=='-'||s[i]=='*'||s[i]=='/')
{
if(s[i+1]!=' ')
continue;
double first = a[--k];
double second = a[--k];
//cout<<first<<" "<<s[i]<<" "<<second<<endl;
switch(s[i])
{
case '+':
a[k]=first+second;
break;
case '-':
a[k]=first-second;
break;
case '*':
a[k]=first*second;
break;
case '/':
if(second==0)
{
cout<<"ERROR\n";
return 0;
}
a[k]=first/second;
break;
}
//cout<<"k="<<k<<"  a[k]="<<a[k]<<endl;
k++;
continue;
}
int tmp=i;
while(s[i]!=' '&&i>=0)
{
i--;
}
int flag=1;
if(s[++i]=='-')
{
flag = -1;
i++;
}else if(s[i]=='+')
{
i++;
}

double num=0;
int tmp0=i;
while(tmp0<=tmp)
{
//cout<<s[tmp0]<<"   ";
if(s[tmp0]=='.')
{
tmp0++;
int point=1;
while(tmp0<=tmp)
{
//cout<<"tmp0:"<<s[tmp0]<<endl;
double re=s[tmp0]-'0';
int pointtmp = point;
while(pointtmp--)
re*=0.1;
point++;
num+=re;
//cout<<num<<endl;
tmp0++;
}
break;
}
num=num*10+(s[tmp0]-'0');
tmp0++;
}
//if(flag<0)
//cout<<num<<"          "<<a[k]<<endl;
a[k++]=num*flag;
//cout<<"i="<<i<<endl;
//cout<<"k="<<k-1<<"  a[k]="<<a[k-1]<<endl;
}

printf("%.1f\n",a[0]);
return 0;
}