PAT (Advanced Level) 1108. Finding Average (20)

#include<cstdio>
#include<cstring>
#include<cmath>
#include<vector>
#include<map>
#include<queue>
#include<stack>
#include<algorithm>
using namespace std;

char s[1000];
int n;
int num,num2,pos;

bool f()
{
int p=0;
for(int i=0;s[i];i++)
{
if(s[i]>='0'&&s[i]<='9') continue;
else if(i==0&&s[i]=='-') continue;
else if(s[i]=='.') {p++; continue;}
else return 0;
}

if(p>1) return 0;

pos=strlen(s);
for(int i=0;s[i];i++) if(s[i]=='.') pos=i;

int hh=strlen(s)-pos-1;
if(hh>2) return 0;

num=0;
for(int i=0;i<pos;i++)
{
if(s[i]>='0'&&s[i]<='9')
num=num*10+s[i]-'0';
if(num>1000) return 0;
}
if(num>1000) return 0;

num2=0;
for(int i=pos+1;s[i];i++)
num2=num2*10+s[i]-'0';

double a=1.0*num+num2/(pow(10.0,strlen(s)-pos-1));
if(a>1000.0) return 0;

return 1;
}

int main()
{
scanf("%d",&n);
double sum=0;
int  geshu=0;
for(int i=1;i<=n;i++)
{
scanf("%s",s);
if(f()==0)
printf("ERROR: %s is not a legal number\n",s);
else
{
double a=0;
a=1.0*num+num2/(pow(10.0,strlen(s)-pos-1));
if(s[0]=='-') a=-a;
sum=sum+a;
geshu++;
}
}
if(geshu==1)
printf("The average of 1 number is %.2lf\n",sum);
else if(geshu==0)
printf("The average of 0 numbers is Undefined\n");
else
printf("The average of %d numbers is %.2lf\n",geshu,sum/geshu);
return 0;
}