巧用sscanf和sprintf：

#include<iostream>
#include<algorithm>
#include<cstring>
#include<string>
#include<cstdio>
#include<cctype>
#include<cmath>
#include<vector>
#include<queue>
#include<set>
#include<map>
#include<stack>
#include<sstream>
#define mm(a,x) memset(a,x,sizeof(a))
#define maxn 100005
using namespace std;
typedef long long ll;
const int INF=0x3f3f3f3f;

int main()
{
   
	int n;	cin>>n;
	int k=0;double sum=0,num;
	while(n--){
   
		getchar();
		char s[maxn];char s1[maxn];
		cin>>s;
		sscanf(s,"%lf",&num);
		sprintf(s1,"%.2f",num);
		bool ans=true;
		for(int i=0;i<strlen(s);i++)
		{
   
			if(s[i]!=s1[i])
			ans=false;
		}
		if(ans&&num>=-1000&&num<=1000){
   
			k++;
			sum+=num;
		}else{
   
			cout<<"ERROR: "<<s<<" is not a legal number\n";
		}
	}
	if(!k) cout<<"The average of 0 numbers is Undefined\n";
	else if(k==1) printf("The average of 1 number is %.2f\n",sum);
	else printf("The average of %d numbers is %.2f\n",k,sum/k);
}
  • 将输入先接收到字符串中
  • 依序检查是否有非数字，注意负号和小数点合法，其他非数字出现即不合法
  • 均为数字后，检查是否有小数点
  • 无小数点则为整数
  • 有小数点要分别检查是否有多个小数点(s.find_first_of(’.’) != s.find_last_of(’.’))和小数位数是否超过2(s.length()-s.find(’.’) > 3)
  • 以上去掉格式不合法数字后，转化为double型，看是否在[-1000,1000]范围内，不在亦不合法
  • 输出时注意区分 0、1(number单数) 及其他 
#include<iostream>
using namespace std;
int main()
{
   
	int n, num = 0;
	cin >> n;
	string s;
	double data, sum = 0;
	for (int i = 0; i < n; i++){
   
		bool islegal = true;
		cin >> s;
		for (int j = 0; j < s.length(); j++){
   
			if ((s[j]>'9' || s[j]<'0') && s[j]!='-' && s[j]!='.'){
   
				cout << "ERROR: " << s << " is not a legal number\n";
				islegal = false;
				break;
			}
		}
		if (islegal){
   
			if (s.find('.') != string::npos){
   
				if(s.find_first_of('.') != s.find_last_of('.') || s.length()-s.find('.') > 3){
   
					cout << "ERROR: " << s << " is not a legal number\n";
					continue;
				}
			}
			data = stod(s);
			if (data >= -1000 && data <= 1000){
   
				sum += data;
				num++;
			}
			else cout << "ERROR: " << s << " is not a legal number\n";
		}
	}
	if (!num) cout << "The average of 0 numbers is Undefined";
	else if (num == 1) printf("The average of 1 number is %.2f", num, sum);
	else printf("The average of %d numbers is %.2f", num, sum/num);
	return 0;
}