ONE LOOP  


for (int i=2; i<= NUMBER_ON_THE_COMMAND_LINE; i++){

     //decide if i is prime on not?
}


SECOND LOOP

for (int i=2; i<=num-1; i++){

    if (number % i == 0){
       //FOUND A FACTOR so number is not prime
       }
}

//didn't find any factors then it is prime


EASY OPTIMIZATION 

to see if a number is prime

//test 2 here 

for (int i=3; i<=num-1; i+=2){

    if (number % i == 0){
       //FOUND A FACTOR so number is not prime
       }
}

//didn't find any factors then it is prime