Need help :: Getting WA :: Quiz 1

[Menon]

#include <iostream>

#include <cstdio>

#include <string>

using namespace std;

int isPrime(int n);

int getMod(string num);

int main()

{

    string num;

    int mod;

    while(cin >> num)

    {

        mod = getMod(num);

        if(mod == 0 || isPrime(mod))

            cout << "Sauron\'s evil spirit!" << endl;

        else

            cout << "You passed the test." << endl;

    }

    return 0;

}

int getMod(string num)

{

    int n = 0;

    for(int i = 0; num[i]; i++)

    {

        n = n * 10 + num[i] - '0';

        n = n % 13;

    }

    return n;

}

int isPrime(int n)

{

    if(n <= 1)

        return 0;

    if(n == 2)

        return 1;

    if(n % 2 == 0)

        return 0;

    for(int i = 3; i * i <= n; i += 2)

    {

        if(i % 2 == 0)

            return 0;

    }

    return 1;

}

[Md. Abul Kalam Azad]

Can anyone hint him to find his bug?

[shahidul.cse.brur]

Menon, your prime checking is wrong ! why you are using

if(i % 2 == 0)

        return 0;

? 
use

if(n % i == 0)

            return 0;

 

[Menon]

if a number is divided by 2 than this number is not a prime number.

so why if( n % 2 == 0 )  this checking is wrong.

[shahidul.cse.brur]

By using -

for(int i = 3; i * i <= n; i += 2)

    {

        if(i % 2 == 0)

            return 0;

    }

you are always checking if i is divided by 2 or not.
Here is two misteke:
1) for loop used here for checking if there any number i by which n%i==0, if there is any value of i, then n is not prime.
    So, i is divisor here, not divined.

2) You always checking if divisible by 2. This is wrong. You should use every number <= sqrt(n) to check whether it can divide n or not.
    so, here divisor should be i not 2.
 Your function is to check whether n is prime or not. So check n%i==0 or not. why you are checking if i%2==0 or not??

For any further query keep posting.

[Menon]

I have got my mistake that was in isPrime ( ) function for loop .accepted code is 

 if(n % i == 0)

            return 0;

instate of 

 if(i % 2 == 0)

            return 0;

tnx Md.Shahidul Islam.