from loop to Nested loops ?

Question

I have this program that returns a factorial of N. For example, when entering 4,,, it will give 1! , 2! , 3!

How could I convert this to use nested loops?

public class OneForLoop
{   
    public static void main(String[] args)
    {
        Scanner input = new Scanner(System.in);     
        System.out.print("Enter a number : ");
        int N = input.nextInt();                
        int factorial = 1;   

        for(int i = 1; i < N; i++)
        {
             factorial *= i;         
             System.out.println(i + "! = " + factorial); 
        }         
    }
}

Answer 1

Rather than just computing everything in a linear fashion, you could consider an inner loop which would do something like what you have in the outer loop. Is that what you are trying to achieve?

Answer 2

If written as nested loops it would look like this:

for (int i = 1; i < N; ++i)
{
    int factorial = 1;
    for (int j = 1; j <= i; ++j) {
         factorial *= j;
    }
    System.out.println(i + "! = " + factorial); 
}

Result:

Enter a number : 10
1! = 1
2! = 2
3! = 6
4! = 24
5! = 120
6! = 720
7! = 5040
8! = 40320
9! = 362880

This program gives the same result as yours, it just takes longer to do so. What you have already is fine. Note also that the factorial function grows very quickly so an int will be too small to hold the result for even moderately large N.

If you want to include 10! in the result you need to change the condition for i < N to i <= N.

Answer 3

Would you consider recursion a nested loop?

public long factorial(int n)
{
    if (n <= 1)
        return 1;
    else
        return n * factorial(n - 1);
}

public static void main(String [] args)
{
    //print factorials of numbers 1 to 10
    for(int i = 1; i <= 10; i++)
        System.out.println(factorial(i));
}

Answer 4

Right now you are calculating your factorial incrementally. Just recalculate it from scratch every time. Be advised that what you have now is better than what I'm posting, but this does follow your requirements.

public class TwoForLoops
{   
    public static void main(String[] args)
    {
        Scanner input = new Scanner(System.in);     
        System.out.print("Enter a number : ");
        int N = input.nextInt();                
        int factorial = 1;   

        for (int i = 1; i < N; ++i)
        {
            factorial = 1;
            for(int j = 1; j <= i; j++)
            {
                 factorial *= j;         
            }
            System.out.println(i + "! = " + factorial);         
        } 
    }
}