1108. Finding Average (20)

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本文介绍了一种算法，用于从一系列输入中筛选出合法的数值，并计算这些数值的平均值。合法数值需位于特定范围内并精确到小数点后两位。

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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

给出若干个字符串，找出其中不是有效数字的字符串，并且求是有效数字的字符串代表的数字的平均数。注意这里的有效数字除了基本的判断（含有不是数字的字符或小数点多于一个）之外，还要是小数点之后不超过两位，且数字在范围[-1000,1000]内。最后还要注意有效数字只有一个的时候，要输出“The average of 1 number ...”，即number不能带s。

代码：

#include <iostream>
#include <cstring>
#include <cstdio>
#include <cstdlib>
#include <vector>
using namespace std;

int main()
{
	int n;
	cin>>n;
	int count=0;
	double sum=0;
	for(int i=0;i<n;i++)
	{
		string s;
		cin>>s;
		bool isvalid=true;
		int point=-1;
		int sign=1;
		int start=0;
		if(s[0]=='-')
		{
			sign=-1;
			start++;
		}
		for(int j=start;j<s.size();j++)
		{
			if(s[j]=='.')
			{
				if(point!=-1) 
				{
					isvalid=false;
					break;
				}
				else
				{
					point=j;
				}
			}
			else if(isdigit(s[j]))
			{
				if(point!=-1&&j-point>2)
				{
					isvalid=false;
					break;
				}
			}
			else
			{
				isvalid=false;
				break;
			}
		}
		if(isvalid)
		{
			double num=atof(s.c_str());
			if(num>1000||num<-1000) printf("ERROR: %s is not a legal number\n",s.c_str());
			else
			{
				sum+=num;
				count++;
			}
		}
		else
		{
			printf("ERROR: %s is not a legal number\n",s.c_str());
		}
	}
	if(count==0)
	{
		printf("The average of 0 numbers is Undefined");
	}
	else
	{
		printf("The average of %d number%s is %.2f",count,count==1?"":"s",sum/count);
	}
}