Produce the sum of the first n values in an array.
function sum(arr, n) {
if (n <= 0 ){
return 0;
} else {
return sum(arr, n-1) + arr[n-1];
}
}
Hello, I have this simple recursive function from freecodecamp. I understand the concept of recursion, however I can't seem to wrap my brain around why this works in this exact use case.
I am mainly struggling to understand how the count is being increased and stored to produce a final total sum, as well as why the function doesn't return 0 as the sum once it hits its breakpoint.
Any explanations are appreciated, thank you.
Also this is not for a project or anything of the sorts, just trying to understand this concept better.
Think of each recursive call as adding a frame on the stack. All the called frames have to give answer before you can compute answer for current frame.
Caller program starts:
FRAME_A
sum([5,7,10],3)
n <= 0: False
answer here = sum([5,7,10],2) + 10
^
wait for this to be computed
let's call this WAIT_A
--------
FRAME_B
sum([5,7,10],2)
n <= 0: False
answer here = sum([5,7,10],1) + 7
^
wait for this to be computed
let's call this WAIT_B
---------
FRAME_C
sum([5,7,10],1)
n <= 0: False
answer here = sum([5,7,10],0) + 5
^
wait for this to be computed
let's call this WAIT_C
---------
FRAME_D
sum([5,7,10],0)
n <= 0: True
return 0
FRAME_D is done.
(Now, we are rolling back)
WAIT_C is now 0. FRAME_C answer is 0 + 5 = 5. FRAME_C is done.
WAIT_B is now 5. FRAME_B answer is 5 + 7 = 12. FRAME_B is done.
WAIT_A is now 12. FRAME_A answer is 12 + 10 = 22. FRAME_A is done.
All frames are done. Final answer = 22.