Recursive Functions

function sum(arr, n) {

if(n<=0){return 0}
else {
   return sum(arr, n-1) + arr[n-1]

HI, i can’t understand the last step,
up to here there are: sum(arr,
but then why

n-1 ) + arr[n-1]

thanks in advance for the answer


Use real numbers. What if I gave you:

sum([7,8], 2);

What will the return statement be on the initial function call. Replace all the variable names with their correct numeric/array values.

return sum(arr, n-1) + arr[n-1]

return sum(arr,1)+arr[1]
return sum(arr,1)+arr[0]

I still do not understand…

There is no need for guessing here. You wrote the function so you should know exactly what the values are :slight_smile:

If 2 is passed in as the second argument then n will equal 2 inside the function. So arr[n-1] will be arr[1]. But what is the actual value of arr[1]? If the array being passed in is [7, 8], then arr equals [7, 8] inside the function. So what would arr[1] be?

Keep going, rewrite return sum(arr,1)+arr[1] by replacing the variables arr and arr[1] with their actual values based on what was passed into the function.


let arr=[7,8];

arr[1] is 8

return sum(arr,1)+8

sum([7,8],1) how does this block work?

essentially, one number is added at a time sum(arr,n-1) where n-1 is just used to decrease the number in the arr to add, and arr[n-1] is the number to add what?

Wel, what would happen if you make that function call?

From what i understand, n-1 in sum(arr, n-1) indicates how many values are gonna be summed from arr…n-1 = 1 in this case so it’s gonna take the first value of arr, that is 7…arr[1] = 8, so 7+8 =15…if arr was e.g [7, 8, 9] and n was 3 then sum([7,8,9], 2) would be 15 and arr[2] would be 9 so 15 + 9 = 24

console.log(sum(arr,1));  //7
console.log(arr[2-1]);   //8

in the second case I understand why it is worth 8 in the first it is not…

What’s the sum of the first 1 numbers in arr?

the first time the sum is 8 and on the second recall the sum becomes +7 ?

because n becomes 1 and 1-1=0 the first element of the array

so arr[n - 1] is used only because in js we start counting from 0 and we pass the number of parameters starting from 1

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