How do I print the factorials of 0-30 on a table

Question

public static void main(String[] args) {

    int n = factorial(30);
    int x = 0;
    while (x <= 30) {
        System.out.println(x + " " + n);
        x = x + 1;
    }


    public static int factorial (int n) {   
       if (n == 0) {
             return 1;
        } else {
            return n * factorial (n-1);
        }
    }
} 

I'm trying to print out something like this:

0 1
1 1
2 2
3 6
4 24
...etc, up to 30 (30!)

What I'm getting instead is this:

0 (30!)
1 (30!)
...etc, up to 30

In words, I'm able to create the left column from 0 to 30 but I want to make it print the factorial of the numbers in the right hand column. With my code, it only prints the factorial of 30 in the right-hand column. I want it to print the factorials in order next to their corresponding number. How can I fix my code to do this?

Answer 1

This is pretty simple. Instead of defining a variable, you call the method with the updated x every time:

System.out.println(x + " " + factorial(x));

Note that your loop could be rewritten as a for loop, which is exactly what they're designed for:

for (int x = 0; x < 30; x++) {
    System.out.println(x + " " + factorial(x));
}

Note a couple of things:

1. The x++ . It's basically a short form of x = x + 1 , though there are some caveats. See this question for more information about that.
2. x is defined in the loop ( for (int x = ... ) not before it
3. n is never defined or used. Rather than setting a variable that's only used once, I directly used the result of factorial(x) .

Note: I'm actually pretty certain that an int will overflow when confronted with 30! . 265252859812191058636308480000000 is a pretty big number. It also overflows long , as it turns out. If you want to handle it properly, use BigInteger :

public BigInteger factorial(int n) {
    if (n == 0) {
        return BigInteger.ONE;
    } else {
        return new BigInteger(n) * factorial(n - 1);
    }
}

Because of BigInteger#toString() 's magic, you don't have to change anything in main to make this work, though I still recommend following the advice above.

Answer 2

As @QPaysTaxes explains , the issue in your code was due to computing the final value and then printing it repeatedly rather than printing each step.

However, even that working approach suffers from a lack of efficiency - the result for 1 computes the results for 0 and 1, the result for 2 computes the results for 0, 1, and 2, the result for 3 computes the results for 0, 1, 2, and 3, and so on. Instead, print each step within the function itself :

import java.math.BigInteger;

public class Main
{

    public static BigInteger factorial (int n) {   
        if (n == 0) {
            System.out.println("0 1");
            return BigInteger.ONE;
          } else {
            BigInteger x = BigInteger.valueOf(n).multiply(factorial(n - 1));
          System.out.println(n + " " + x);
            return x;
          }
      }
  public static void main(String[] args)
  {
    factorial(30);
  }
}

Of course, it would be faster and simpler to just multiply in the loop :

import java.math.BigInteger;

public class Main
{
  public static void main(String[] args)
  {
    System.out.println("0 1");
    BigInteger y = BigInteger.ONE;
    for (int x = 1; x < 30; ++x) {
        y = y.multiply(BigInteger.valueOf(x));
        System.out.println(x + " " + y);
    }
  }
}

Just for fun, here's the efficient recursive solution in Python:

def f(n):
    if not n:
        print(0, 1)
        return 1
    else:
        a = n*f(n-1)
        print(n, a)
        return a

_ = f(30)

And, better still, the iterative solution in Python:

r = 1
for i in range(31):
    r *= i or 1
    print(i, r)