1108 Finding Average （20 分）

The basic task is simple: given N real numbers, you are supposed to calculate their average. But what makes it complicated is that some of the input numbers might not be legal. A legal input is a real number in [−1000,1000] and is accurate up to no more than 2 decimal places. When you calculate the average, those illegal numbers must not be counted in.

Input Specification:

Each input file contains one test case. For each case, the first line gives a positive integer N (≤100). Then N numbers are given in the next line, separated by one space.

Output Specification:

For each illegal input number, print in a line ERROR: X is not a legal number where X

 is the input. Then finally print in a line the result: The average of K numbers is Y where K is the number of legal inputs and Y is their average, accurate to 2 decimal places. In case the average cannot be calculated, output Undefined instead of Y. In case K is only 1, output The average of 1 number is Y instead.

Sample Input 1:

7
5 -3.2 aaa 9999 2.3.4 7.123 2.35

Sample Output 1:

ERROR: aaa is not a legal number
ERROR: 9999 is not a legal number
ERROR: 2.3.4 is not a legal number
ERROR: 7.123 is not a legal number
The average of 3 numbers is 1.38

Sample Input 2:

2
aaa -9999

Sample Output 2:

ERROR: aaa is not a legal number
ERROR: -9999 is not a legal number
The average of 0 numbers is Undefined

非法數字形式樣例中已經全部給出，注意只有一個合法數字的輸出形式。還有注意sscanf的使用!!!

#include <iostream>
#include <cstdio>
using namespace std;

bool Jundge(string str,double &x){
	int i=0,n1 = 0,n2 = 0;
	if(str[i]=='-')i++;
	for(;i<str.size();i++){
		if(str[i]=='.'){
			n1++;
			if(n1 >= 2)
				return false;
		}
		else if(str[i]>='0' && str[i]<='9'){
			if(n1)n2++;
		}else
			return false;	
	}
	if(n2 > 2)
		return false;
	sscanf(str.c_str(),"%lf",&x);//注意x要取地址
	if(x>=-1000 && x<=1000)
		return true;
	else
		return false;
}

int main()
{
    int n,cnt = 0;
    cin>>n;
    string str;
    double res = 0,x = 0;
    for(int i=0;i<n;i++){
        cin>>str;
        if(Jundge(str,x)){
        	res += x;
        	cnt++;
        }else{
        	printf("ERROR: %s is not a legal number\n",str.c_str());
        }
    }
    if(cnt == 0)
        printf("The average of 0 numbers is Undefined");
    else if(cnt == 1)
        printf("The average of 1 number is %.2f",res);
    else
        printf("The average of %d numbers is %.2f",cnt,res/cnt);

    return 0;
}