Recursion (count)

[Embraz]

function countup(n) {
  if (n < 1) {
    return [];
  } else {
    const countArray = countup(n - 1);
    countArray.push(n);
    return countArray;
  }
}
console.log(countup(5));

Please I don’t understand how this returns [1,2,3,4,5] instead of [4,3,2,1] as I am thinking cos the number ’ n ‘ ought to be decreasing (n-1)

[JeremyLT]

Two questions for you to think about

1. What is countup(4) supposed to return

2. What happens if you push 5 onto that result

[Embraz]

I still don’t understand this. Please Can you explain the code in your own words. Thanks

[Embraz]

The logic please using n = 5. Thank you

[JeremyLT]

Can you please try to answer the two questions I asked?

[mfahadiqbal]

I understand that the code may seem confusing at first. Let’s break it down step by step.

The countup function takes a parameter n and returns an array of numbers starting from 1 up to n.

Here’s how it works:

1. When countup is called with a value of n, it checks if n is less than 1 (n < 1).
2. If n is less than 1, it means we’ve reached the base case, and the function returns an empty array [].
3. If n is not less than 1, it means we need to continue counting up.
4. In this case, the function recursively calls itself with n - 1 as the argument: countArray = countup(n - 1).
5. The recursive call will continue until it reaches the base case, where n is less than 1.
6. Once the base case is reached, the recursive calls start returning, and each call adds its respective n value to the countArray.
7. Finally, the function returns the countArray, which now contains all the numbers from 1 to n in ascending order.

So, when you call countup(5), it goes through the recursive calls as follows:

1. countup(5) calls countup(4)
2. countup(4) calls countup(3)
3. countup(3) calls countup(2)
4. countup(2) calls countup(1)
5. countup(1) returns an empty array []
6. countup(2) adds 2 to the array and returns [2]
7. countup(3) adds 3 to the array [2] and returns [2, 3]
8. countup(4) adds 4 to the array [2, 3] and returns [2, 3, 4]
9. countup(5) adds 5 to the array [2, 3, 4] and returns [2, 3, 4, 5]

That’s why the output is [1, 2, 3, 4, 5] instead of [4, 3, 2, 1].

I hope this explanation helps clarify how the code works. If you have any more questions, feel free to ask!

[Embraz]

Just the explanation I needed. I’m immensely grateful. Thank you very much. I now understand the logic

[Embraz]

Thanks you. I think I’ve understood it

[Embraz]

Thank you. I think I’ve understood it. I’m grateful