CareerCup

bool IsPrime(int N){
if(<=0)return false;
if(N<=3)return true;

for(int i=2;i<N;i++) {
if( (N/i)*i = N)return true;
}
return false;
}

bool IsPrime(int N)
{
if(N<0 || N%2==0)
return false;
for(int i =3;i<N;i=i+2)
{
if((N/i)*i==N)
return true;
}
return false;
}

}

>> for(int i =3;i<N;i=i+2)

You need not check till N it is ok to check till sqrt(N)

Did not get. "if((N/i)*i==N)" isn't this always true?

Is it not N%i==0 ?

bool isPrime( long num )
{
  if (num < 2) return false;
  if (num < 4) return true;
  if (num % 2 == 0) return false;
  long i, x = num;
  int count = 0;
  for (i = 3; i < x; i+=2)
  {
    if((num % i) == 0) return false;
    if (count++ == 100)
    {
      count = 0;
      x = num/i; // we can do this in every iteration, but to speed up let us do it every 100th iteration.
    }
  }
  return true;
}

#include<iostream.h>
#include<math.h>
int prime(int n);

int main(){
int n;
cout << "Enter N:";
cin >> n;

cout << "Prime:" << prime(n) << "\n";
return 0;
}

int prime(int n){
int i = 2;
int l = int(sqrt(n));
int flag = 1;
while( i <= l ){
if ( n%i == 0 ){
flag = 0;
break;
}
++i;
}
return flag;
}

My code :

unsigned char is_num_prime(int num)
{
    if(num == 1)
        return 0;
    else if(num >= 2 && num <= 3)
        return 1;
    else if(num % 2 == 0)
        return 0;
    else
    {
        int i = 3;
        float fact = ((float)num)/i;
        while(fact > 1.0f)
        {
            if(num % i == 0)
                return 0;
            i += 2;
            fact = ((float)num)/i;
        }
    }
    
    return 1;
}