# PAT甲级 1108 Finding Average (20分)

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
``````

# 注意点

• 字符串数组不能开太小，否则会造成 “运行时错误”，即数组越界。实测大小至少为50.

# 方法一

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

# 方法二

``````#include <cstdio>
#include <cstring>
#include <string>
using namespace std;

int main() {
int n, cnt = 0;
scanf("%d", &n);
char str;
double total = 0;
for (int i = 0; i < n; i++) {
scanf("%s", str);
int len = strlen(str), index = -1;
bool flag = true;
for (int i = 0; i < len; i++) {
if (str[i] == '.') {
if (i < len - 3 || index != -1) {
flag = false;
break;
}
index = i;
} else if ((str[i] < '0' || str[i] > '9') && !(i == 0 && str[i] == '-')) {
flag = false;
}
}
if (!flag) {
printf("ERROR: %s is not a legal number\n", str);
} else {
double num = stod(str);
if (num < -1000 || num > 1000) {
printf("ERROR: %s is not a legal number\n", str);
} else {
cnt++;
total += num;
}
}
}
if (cnt == 0) {
printf("The average of 0 numbers is Undefined");
} else if (cnt == 1) {
printf("The average of 1 number is %.2f", total / cnt);
} else {
printf("The average of %d numbers is %.2f", cnt, total / cnt);
}
return 0;
}
``````

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