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Reputation: 1438

What's going on inside of this recursive function?

I have this recursive factorial function:

public class Driver{
public static void main (String args[])
{

    System.out.println(factorial(5));

}

private static int factorial(int n)
{
    int product;
    if (n <= 2){ product = n; return 2;}
    else{
        product = n * factorial(n-1);
        System.out.println("current product =" + product);
        System.out.println("product = " + n + " * factorial(" + (n-1) + ")");
    }
    return product;
}
}

It prints the following:

current product =6
product = 3 * factorial(2)
current product =24
product = 4 * factorial(3)
current product =120
product = 5 * factorial(4)
120

I'm trying to figure out what the heck is going on here. I don't understand how it starts printing for n = 2. And where did current product = 6 come from? Thanks!

Upvotes: 1

Views: 63

Answers (1)

Karthik T
Karthik T

Reputation: 31952

n==2 is your base case, so the first function to get past the below line is the one where n-1 == 2 or n == 3. This is because your output is performed after the recursive call, so it prints the deepest call first (thanks Patricia Shanahan) . If you want it to print from big to small, move your println lines above the line which calls factorial recursively. 

product = n * factorial(n-1);

Also, 

if (n <= 2){ product = n; return 2;}

This is wrong for so many reasons..

It should be 

if (n <= 1){ return 1;}

If you are having trouble understanding this, remember that each call of factorial is independent of other calls.

Upvotes: 4

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