# 【題解】ZeroJudge e795: p2.質數日

【題目敘述】https://zerojudge.tw/ShowProblem?problemid=e795
【解題想法】題目保證不會有非日期表示之輸入，只要判斷是否為質數。

```#include <iostream>
#include <cmath>
using namespace std;
const int maxn = 30000000;
bool p[maxn];

int main() {
// 建立指數表
p[0] = p[1] = 1;
for (int i=2; i<maxn; i++){
if (!p[i]){
for (int j=2*i; j<maxn; j+=i){
p[j] = 1;
}
}
}
int D;
cin >> D;
while (D--){
int d;
cin >> d;
bool flag = true;
for (int i=8; i>=1; i--){
int tmp=d, b = 1;
for (int j=1; j<=i; j++) b *= 10;
tmp %= b;
if (p[tmp]){
flag = false;
break;
}
}
if (flag) cout << d << " is a Prime Day!\n";
else cout << d << " isn't a Prime Day!\n";
}
return 0;
}
```

Python code (credit: Amy Chou)

```def isPrime(x):
for i in range(2, int(pow(x, 0.5))):
if x % i == 0:
return False
return True

D = int(input())
for _ in range(D):
d = int(input())
flag = True
for i in range(8, 0, -1):
tmp = d % pow(10, i)
if not isPrime(tmp):
flag = False
break
if flag:
print(f"{d} is a Prime Day!")
else:
print(f"{d} isn't a Prime Day!")
```