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 1:75 -3.2 aaa 9999 2.3.4 7.123 2.35
Sample Output 1:ERROR: aaa is not a legal numberERROR: 9999 is not a legal numberERROR: 2.3.4 is not a legal numberERROR: 7.123 is not a legal numberThe 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

#include 
#include 
using namespace std;

const double NOTANUMBER = 9999.9999;

double isNum(string str){

    bool isNegative = false;
    bool isExist = false;
    bool isError = false;
    int decimal = 0;
    double degree = 10;
    double sum = 0;
    for(int i=0; i='0')
        {
            if(!isExist)
                sum = sum*degree + (str[i]-'0');
            else
            {
                sum = sum+(str[i]-'0')*degree;
                degree = degree * 0.1;
                decimal++;
                if(decimal>2)
                    isError = true;
            }
        }
        else
            isError = true;
        if(isError) break;
    }

    if(sum>1000)
        isError = true;
    if(isError)
    {
        cout << "ERROR: " << str << " is not a legal number" <> N;
    for(int i=0; i> str;
        temp = isNum(str);
        if( temp != NOTANUMBER )
        {
            count++;
            sum+=temp;
        }

    }
    cout << "The average of " << count;
    if(count == 1)
        cout << " number is ";
    else
        cout << " numbers is ";
    if(count == 0)
        cout << "Undefined";
    else
        printf("%.2lf",sum/count);
    return 0;
}