Recursion: trying to break it down

JavaScript
1

Hi guys,

I am grappling with recursion now, and since this is a super difficult topic for me, I’m trying to break down how exactly the code in this challenge works.

The thing is I managed to complete the challenge but I’m still not sure I understand how it works.

The example code in the challenge goes like this:
[EDIT: correction, previously the code snippet below contained a for loop instead of the recursive function)

function multiply(arr, n) {
    if (n <= 0) {
      return 1;
    } else {
      return multiply(arr, n - 1) * arr[n - 1];
    }
  }

Below you can see my attempt at breaking down the code with some example variables. There are questions that I am not able to answer yet, and I was hoping that some of you would be able to shed some light on them.

var arr = [1, 2, 3, 4, 5]
var n = 2 (base index is zero?)

multiply(arr, n)

We begin when n is 2.
Q: How does the function know that n is 2?
A: I have just fed it that as the second argument “var n”

First round: n is 2. It is not less than or equal to zero, so we go to the else statement:

multiply(arr, n - 1) * arr[n - 1]
^this is where we call ^ this is not the
the function           function any more.
                       This is how we get
                       to the desired
                       element of the array
                       by referring to its index.

This means: take the array, subtract 1 from n and multiply it by the element in the array whose index is [n minus 1].

n is now [2 - 1], so = 1.
We multiply it by the array’s element whose index is [2 - 1], which is the number 2 (keeping in mind that the array’s index and the number which sits there are different things)

So we do 1 * 2. Result: 2

Q: What and where is now “result: 2”? Where is it kept? And how does the function know it should call itself again? Where is the built-in loop-like mechanism that I fail to see?

Aren’t “else statement” and “return” the points where functions terminate?

2

hello there, recursion is some needle in the flesh for me too, however i read somewhere that recursion uses something called a stack, where it first “stacks” values to be operated on in order, and after doing so begins working with on the else or return statement, so the place where you wrote

Blockquote First round: n is 2. It is not less than or equal to zero, so we go to the else statement:

happens after the stack is called, ie the multiplying of the elements. The function knows when to stop calling itself when the n - 1 is equal to 0

3

Taking the example you have given above, the function first gets n, if n is greater than 0, it adds n to the stuck then goes to n-1 and checks whether it is greater than 0. Example where n = 3 and the array is [1, 2, 3, 4]…
the stack will be as follows: 3, 2, 1 ( assume they are lying from bottom to top, where 3 is at the bottom and 1 is at the top, this is stacking)
after the stack has been built, the return statement is now executed.
Thats the best i can do in explaining it. Hope it helps