Factorial using Recursion in Java

Question

I am learning Java using the book Java: The Complete Reference. Currently I am working on the topic Recursion.

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Answer 1

First you should understand how factorial works.

Lets take 4! as an example.

4! = 4 * 3 * 2 * 1 = 24

Let us simulate the code using the example above:

int fact(int n)
    {
        int result;
       if(n==0 || n==1)
         return 1;

       result = fact(n-1) * n;
       return result;
    }

In most programming language, we have what we call function stack. It is just like a deck of cards, where each card is placed above the other--and each card may be thought of as a function So, passing on method fact:

Stack level 1: fact(4) // n = 4 and is not equal to 1. So we call fact(n-1)*n

Stack level 2: fact(3)

Stack level 3: fact(2)

Stack level 4: fact(1) // now, n = 1. so we return 1 from this function.

returning values...

Stack level 3: 2 * fact(1) = 2 * 1 = 2

Stack level 2: 3 * fact(2) = 3 * 2 = 6

Stack level 1: 4 * fact(3) = 4 * 6 = 24

so we got 24.

Take note of these lines:

result = fact(n-1) * n;
           return result;

or simply:

return fact(n-1) * n;

This calls the function itself. Using 4 as an example,

In sequence according to function stacks..

return fact(3) * 4;
return fact(2) * 3 * 4
return fact(1) * 2 * 3 * 4

Substituting results...

return 1 * 2 * 3 * 4 = return 24

I hope you get the point.