Replacing Loops using Recursion

JavaScript
1

Tell us what’s happening:
can any one tell me whats happening here.
i’ve read the topic and am still not clear about recursion.
@caryaharper

  **Your code so far**


function sum(arr, n) {
// Only change code below this line
if( n <= 0){
return 1;
}
else
{
return sum(arr, n - 1) + arr[n - 1]
}

// Only change code above this line
}

sum([1, 2] , 2);
console.log()

  **Your browser information:**

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/90.0.4430.93 Safari/537.36.

Challenge: Replace Loops using Recursion

Link to the challenge:

2

In your if you should be returning 0.

The recursion in this case can be a bit confusing as it seems a bit backwards compared to a loop, but written out it kinda looks like this:

/*

sum([1,2], 2) returns sum(arr, 2 - 1) + arr[2 - 1]
sum(arr, 1) returns sum(arr, 1 - 1) + arr[1 - 1] 

this sum(arr, 1 - 1) returns 0 so sum(arr, 1) returns 
0 + arr[0] == 1

therefore

sum([1,2], 2) returns sum(arr, 1) + arr[1] 
and we now know that sum(arr, 1) returns 1 so
sum([1,2], 2) returns 1 + arr[1] or 1 + 2 == 3

*/

As you can see you have to get all the way to the bottom in order to start building your answer up. Doing recursion in the way above will quickly cause a RangeError for exceeding the maximum call stack size so do be careful if you are trying to use this method over a loop.

1 Like
3

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