PAT 1108 Finding Average (20)

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

题目大意：判断输入的字符串是否在[-1000, 1000] ，并且小数点不超过2位； 计算合法数字的平均数；
思路：用sscanf()把输入的字符串转化为数字temp，在用sprintf()把temp按照两位小数转化为字符串b; 在a和b的长度范围内判断两个字符串的内容是否一致，如果不一致肯定不是合法的数字； 此外还可能存在这一的情况
     a对于的数字没有小数部分，当把temp转换为b的时候，会在b后面添加".00"； 用上面的方法判断就会出错；对于这种情况， 如果b的长度比a小，则肯定不是合法数字 

#include<iostream>
#include<cctype>
#include<string>
#include<vector>
#include<cstring>
using namespace std;
int main(){
    int n, i, cnt=0;
    cin>>n;
    double temp=0.0, sum=0.0;
    for(i=0; i<n; i++){
        char a[60], b[60];
        scanf("%s", a);
        sscanf(a, "%lf", &temp);
        sprintf(b, "%.2f", temp);
        int flag=1;
        for(int j=0; j<strlen(b) && j<strlen(a); j++){
            if(a[j]!=b[j]){
                flag=0;
                break;
            }
        }
        if(strlen(a)>strlen(b)) flag=0;
        if(flag && (temp<=1000 && temp>=-1000)) {
            sum += temp;
            cnt++;
        }
        else printf("ERROR: %s is not a legal number
", a);
    }
    if(cnt>1) printf("The average of %d numbers is %.2f
", cnt, sum/(cnt*1.0));
    else if(cnt==1) printf("The average of %d number is %.2f
", cnt, sum);
    else printf("The average of 0 numbers is Undefined
");
return 0;}