I am trying to take the input from the user for a range and then finding all the prime numbers in that range. I am using the logic that any nuber that is greater than 2 and less than itself, which is not divisible by any number in this range (2 to less than itself) is a prime number.
public class Main
{
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.print("Enter the range : ");
int n1=sc.nextInt();
int n2=sc.nextInt();
int fl=0;
for(int i=n1;i<n2;i++)
{
for(int j=2;j<i;j++)
{
if(i % j == 0)
{
fl=1;
}
}
if(fl == 0)
System.out.println(i);
}
}
}
This is the output I am getting: 1 2 3
When the range is 1 to 20 (No more than 3). Please help me out.
As pointed out by Turamarth, you need to reset the flag at each iteration. I also suggest you to loop until i <= n2, otherwise you would miss the last number, and add an input check.
public static void main(String args[])
{
int n1;
int n2;
boolean flag;
Scanner sc = new Scanner(System.in);
System.out.print("Enter the range: ");
n1 = sc.nextInt();
n2 = sc.nextInt();
sc.close();
if (n1 < 2)
n1 = 2;
if (n2 < 2)
{
System.out.println("No prime numbers in range.");
return;
}
System.out.println("Result: ");
for(int i = n1; i <= n2; i++)
{
flag = false;
for(int j = 2; j < i; j++)
{
if(i % j == 0)
{
flag = true;
}
}
if(!flag)
System.out.println(i);
}
}
Example with [1, 19]:
Enter the range: 1 19
Result:
2
3
5
7
11
13
17
19
Consider that:
i % j == 0), by putting a break; statement inside the if condition;i >= 3 you can increase it by 2: i += 2;j < i, you can just test up to j * j <= i (equivalent to j <= sqrt(i)). In fact, there's no need to check divisors greater than the square root of i, as any larger divisor would correspond to a smaller one already tested. For example, if the number to check is 91, we just need to test up until sqrt(91) = 9.539 ~= 9, so 2, 3, 5 and 7. We don't check 13 and the others, because their corresponding divisors (in this case, for 13 is 7, because 7 * 13 = 91) would have been already checked.