2399：GPA

Problem Description Each course grade is one of the following five letters: A, B, C, D, and F. (Note that there is no grade E.) The grade A indicates superior achievement , whereas F stands for failure. In order to calculate the GPA, the letter grades A, B, C, D, and F are assigned the following grade points, respectively: 4, 3, 2, 1, and 0.

Input The input file will contain data for one or more test cases, one test case per line. On each line there will be one or more upper case letters, separated by blank spaces.

Output Each line of input will result in exactly one line of output. If all upper case letters on a particular line of input came from the set {A, B, C, D, F} then the output will consist of the GPA, displayed with a precision of two decimal places. Otherwise, the message "Unknown letter grade in input" will be printed.

Sample Input A B C D F B F F C C A D C E F

Sample Output 2.00 1.83 Unknown letter grade in input

分析：

即使提示“Unknown letter grade in input”的信息，仍能进行后续输入，而不是程序到此结束。

代码如下：

#include <iostream>
#include <iomanip>
#include <string>
using namespace std;

int main()
{
	char grade;
	string str;
	while (getline(cin,str))//getline按行读取
	{
		int sum = 0,k = 0,sign = 0;
		int i = 0;
		while (i < str.length())
		{
			grade = str[i++];
			switch (grade)
			{
			case 'A':
				sum += 4;
				k++;
				break;
			case 'B':
				sum += 3;
				k++;
				break;
			case 'C':
				sum += 2;
				k++;
				break;
			case 'D':
				sum += 1;
				k++;
				break;
			case 'F':
				sum += 0;
				k++;
				break;
			case ' ':
				break;
			default:
				sign = 1;
				break;
			}
		}
		if(sign == 1)
			cout << "Unknown letter grade in input" << endl;
		else
			cout << fixed << setprecision(2) << (sum * 1.0 / k) << endl;//设置小数点后的精度为2
	}

	return 0;
}