Hi I am new to C++
can some one tell me how to make my code faster

#include
#include
#include<math.h>
using namespace std;

int prime_generator( const int &num1, const int &num2)
{
int ctr=0;
for(int i=num1;i<=num2;i++)
{

	   for(int j=2;j<=sqrt(i);j++)
           {
            if(i%j==0)
              ctr++;
           }
           if(ctr==0&&i!=1)
           {
		   	printf("%d \n",i);
	
		    fflush(stdout);
            
			ctr=0;
           }
           ctr=0;
           
         
   }

return 0;
}

int main() {

int num1=0,num2=0,t=0;
printf("Input:\n");
scanf("%d",&t) ; 

pair<int,int> myvector[10];

for(int j=0;j<t;++j){

scanf("%d %d",&num1,&num2) ;

   `

myvector[j].first=num1 ; //,num2));
myvector[j].second=num2;

}

printf("Output:\n");

for(int j=0;j<t;++j){

	printf("\n");

prime_generator(myvector[j].first,myvector[j].second);

}

return 0;
}

Sqrt(i) i think is calculated every time loop finishes

can you explain what do you mean by " You don’t need to count the factors, just determine whether there are any, and you only need to test division by one even number if you choose the right one"