Replace Loops using Recursion - Why is the answer 5!? Please help

JavaScript
1

Tell us what’s happening:
I have spent a full day and nearly all night trying to figure out how this algorithm works.

I just need a textual breakdown of how this algorithm outputs 5, for me 6 makes sense as it’s every number in the array -1 then added together, so 2-1, 3-1, 4-1, so 1+2+3. But that clearly isn’t the case, some one please help, I’m close to defeat!

Your code so far


function sum(arr, n) {
// Only change code below this line
if (n <= 0) {
  return arr[0];
} else {
  return sum(arr, n - 1) + arr[n];
}
// Only change code above this line
}
var total = sum([2, 3, 4], 1);
console.log(total)

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:71.0) Gecko/20100101 Firefox/71.0.

Challenge: Replace Loops using Recursion

Link to the challenge:
https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/basic-javascript/replace-loops-using-recursion

1 Like
2

it is asking to sum the numbers till index 1 in the array, so to sum arr[0] and arr[1]

notice that n is always only used as array index, so it is the array index that is decreased not the value in the array

1 Like
3

n is the index of the highest value you should sum. 2+3 is 5.

1 Like
4

Your code has been blurred out to avoid spoiling a full working solution for other campers who may not yet want to see a complete solution. In the future, if you post a full passing solution to a challenge and have questions about it, please surround it with [spoiler] and [/spoiler] tags on the line above and below your solution code.

Thank you.

5

Thankyou so much for your reply, it was really helpful. Could you please review my comments at the bottom of my code here and tell me if I am understanding this correctly…?
Sorry about the spoiler, I have [spoiler] commented out my reply!
Thanks again.

function sum(arr, n) {
// Only change code below this line
if (n <= 0) {
return arr[0];
} else {
return sum(arr, n-1) + arr[n];
}
// Only change code above this line
}
var total = sum([2, 3, 4], 1);
console.log(total)

/* So…
We pass ‘1’ into the function sum([2, 3, 4], “1”<-)
Because it is not <= 0, the function returns the array (‘arr’), decrements ‘n’/index by 1 (meaning it is now 0). But before returning, asks the function to add the contents of ‘arr’ together (+ arr[n]), up until the index specified index [n], 1 in this case?

So it would look kind of like this,

sum([2,3,4],1)
sum([0],0) + [2+3] = 5

Feel free to make any corrections to my workings at here to give a more acurate depiction of what happens within the function. Thankyou so much…
*/

6

Thankyou very much ArielLeslie, both yours and Ieahleen’s comments helped to to give me a better understanding! Good work!

7

the function doesn’t return arr

"recursion breakdown

the function sum([2,3,4], 1) is called
n is higher than 0 so this function returns the sum of an other function and a number, but n is decreased by 1: sum([2,3,4], 0) + 3
the function is called, this time n is 0 so the function returns arr[0], which is substituted instead of the function call
so sum([2,3,4], 0) becomes 2
and sum([2,3,4], 0) + 3 becomes 2+3 so 5

1 Like
8

Thankyou so much, really appreciate the help!