题目
he 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
思路
使用stringstream实现字符串向double的转化，如果失败会返回false，这样就能判断是否为合法float，用法如下：
string str;
if (!getline(cin, str))
{// error: didn't get any input
}
istringstream ss(str);
float f;
if (!(ss >> f))
{// error: didn't convert to a float
}
如果是float，还需判断是否在范围-1000~1000内，以及小数点后是否仅有2位：
if (str.find('.')!=string::npos && str.size()-str.find('.')>3){flag = false; }
注意不要忽略前面的str.find('.')!=string::npos。
代码
#include <iostream>
#include <sstream>
#include <iomanip>
using namespace std;int main() {int n;cin >> n;double sum = 0;int legal = 0;string str;double temp;for (int i=0; i<n; i++){bool flag = false;cin >> str; stringstream ss(str);if (ss>>temp && temp>=-1000 && temp<=1000){ flag = true;if (str.find('.')!=string::npos && str.size()-str.find('.')>3){flag = false; }}if (flag){legal++;sum += temp;}else{cout << "ERROR: " << str << " is not a legal number" << endl;}}if (legal==0){cout << "The average of 0 numbers is Undefined" << endl;}else if (legal==1){cout << "The average of 1 number is " << setprecision(2) << fixed << sum << endl;}else{cout << "The average of "<< legal << " numbers is " << setprecision(2) << fixed << sum / legal<< endl;}return 0;
}