原题链接：PAT 1108 Finding Average

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

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题目大意：

给出一组数，这组数里面存在不合法的值
判断每个数是否合法，以及输出合法数的平均值

方法一：模拟

C++ 代码：

// pat_a_1108#include<iostream>
using namespace std;int main(){int n;cin >> n;int cnt = 0;    // 合法的数的个数double sum = 0;while(n--){string num;cin >> num;double x;bool success = true;    // 是否合法的标志try{size_t idx; // 记录用了几个字符x = stof(num, &idx);    //字符串转化为浮点型if(idx < num.size()) success = false;   //类似与5.2abc 不合法}catch(...){ //表示抛出任何异常success = false;}if(x < -1000 || x > 1000) success = false;int k = num.find('.');  //找到小数点if(k != -1 && num.size() - k > 3) success = false;if(success) cnt ++ , sum += x; else printf("ERROR: %s is not a legal number\n",num.c_str());}// printf("%d\n",cnt);if(cnt > 1) printf("The average of %d numbers is %.2lf\n", cnt, sum / cnt);else if(cnt == 1) printf("The average of 1 number is %.2lf\n", sum);else puts("The average of 0 numbers is Undefined");return 0;
}