PTA乙级 （*1054 求平均值 (20分)）

#include <cstdio>
#include <cstring>
#include <string>
#include <iostream>
#include <cmath>
#include <algorithm>
#include <iomanip>
using namespace std;
bool check(string &str)
{
	bool flag=false;
	int i=0,a=0,b=0;
	if(str[i]==‘-‘) i++;
	for(;i<str.size();i++)
	{
		if((!isdigit(str[i]))&&(str[i]!=‘.‘)) return false;
		if(str[i]==‘.‘) 
		{
			a++;
			flag=true;
			continue;
		}
		if(flag) b++;
	}
	if(a>1||b>2||stod(str)>1000||stod(str)< -1000) return false;
	else return true;
}
int main()
{
	int n,count=0;
	string str;
	double sum=0;
	cin>>n;
	for(int i=0;i<n;i++) 
	{
		cin>>str;
		if(!check(str)) cout<<"ERROR: "<<str<<" is not a legal number\n";  
		else if(check(str)){
			count++;
			sum+=stod(str);
		}
	}
	if(count==0) cout<<"The average of 0 numbers is Undefined\n";
	else if(count==1) cout<<"The average of 1 number is "<<fixed<<setprecision(2)<<sum<<endl;
	else printf("The average of %d numbers is %.2f\n",count,sum/count);
	return 0;
}