Decrement Not Passing Test in Recursion Exercises. Why?

[xorezx]

Tell us what’s happening:

I don’t understand why decrement doesn’t work: (n–) doesn’t pass test, (n-1) does.

E.g., this works:

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

This doesn’t:

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

Your code so far

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

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Challenge: Replace Loops using Recursion

Link to the challenge:

freeCodeCamp.org

[JeremyLT]

Hi there!

@xorezx

n--

The decrement operator changes the value of n, so you have, in essence,

@xorezx

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

[kevinSmith]

There are two things at work here.

First of all, the post decrement operator evaluates and then decrements. If you put the -- in front, it will decrement and then evaluate. It’s a subtle but occasionally very important difference.

Secondly, you are using it twice so it is performing that decrement twice. You could do else return sum(arr, --n) + arr[n]; and that would work, but would be difficult to read code.