A1108. Finding Average

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

#include<cstdio>
#include<iostream>
#include<string.h>
using namespace std;
char num[100];
const double INF = 100000000.0;
double prase_(char num[]){
    int len = strlen(num);
    int pos = -1, cnt = 0;
    int tag = 0;
    for(int i = 0; num[i] != '\0'; i++){
        if(!(num[i] >= '0' && num[i] <= '9' || num[i] == '.' || num[i] == '-')){
            return INF;
        }
        if(num[i] == '.')
            cnt++;
        if(cnt > 1)
            return INF;
    }
    if(num[0] == '-')
        tag = 1;
    for(pos = 0; num[pos] != '.' && num[pos] != '\0'; pos++);
    int P = 1, ansL = 0, ansR = 0;
    if(pos == '\0'){
        for(int i = len - 1; i >= tag; i--){
            ansL += (num[i] - '0') * P;
            P *= 10;
        }
        if(tag == 1)
            return ansL * -1.0;
        else return ansL * 1.0;
    }else{
        if(len - pos - 1 > 2)
            return INF;
        P = 1; ansL = 0;
        for(int i = pos - 1; i >= tag; i--){
            ansL += (num[i] - '0') * P;
            P *= 10;
        }
        P = 1; ansR = 0;
        for(int i = len - 1; i > pos; i--){
            ansR += (num[i] - '0') * P;
            P *= 10;
        }
        double temp = ansR * 1.0;
        for(int i = 0; i < len - pos - 1; i++){
            temp *= 0.1;
        }
        if(tag == 1)
            return ((double)ansL + temp) * -1.0;
        else return (double)ansL + temp;
    }
}
int main(){
    int N, cnt = 0;
    double sum = 0, temp = 0;
    scanf("%d", &N);
    for(int i = 0; i < N; i++){
        scanf("%s", num);
        temp = prase_(num);
        if(temp > 1000.0 || temp < -1000.0){
            printf("ERROR: %s is not a legal number\n", num);
        }else{
            sum += temp;
            cnt++;
        }
    }
    if(cnt > 1){
        double prt = sum / (double)cnt;
        printf("The average of %d numbers is %.2f\n", cnt, prt);
    }else if (cnt == 0){
        printf("The average of 0 numbers is Undefined\n");
    }else if(cnt == 1){
        double prt = sum / (double)cnt;
        printf("The average of %d number is %.2f\n", cnt, prt);
    }
    cin >> N;
    return 0;
}