Reduce giving a weird result

JavaScript
1

ive been better understanding reduce un-nesting arrays turning arrays into objects getting a feel for how it operates however i found a cool artical on freecodecamp and am going though the examples and one seems to acting strangly

let arr = [2,4,6,8]
const newArr = arr.reduce((total, amount, index, array) => {
  total += amount
  return total/array.length
}, 0);     // comes to 2.4453125

it comes to a werird number instead of 5 why is this? the strange thing is ive been going though the functional programming part of the course again and yesturday a calucated an avg using reduce without an issue. i console.loged array.length and it is 4 and if i take it off the total comes to 20 so i have no idea why its behaving like this for me i also tried another online editor just to check.

2

First, array.length is 4

Now, first iteration: total is 0, amount is 2. The callback returns 2/4, and this is the total for next iteration
Now total is 0.5 and amount is 4, the callback returns 4.5/4. This is the total for next iteration.
Etc

If you want to have the division inside the reduce method, be sure to not divide total by it as it was already divided in previous iteration

3

oh gwd soon as i read this i see being very dumb due to using => { return } rather just 1 line arrow syntax sorry!

4

Arrow syntax is unrelated, it depends on the order of operation you are using

5

i must be confusing myself really badly

6

You want to divide the total (after all the sum is done) by the array length, what you are doing instead is adding once then dividing the subtotal

((((0+2)/4 + 4)/4 + 6)/4 + 8)/4

If you want to make the division inside the reduce method a thing you can do is equivalent to this:
((((0 + 2/4) + 4/4) + 6/4) + 8/4)

Or just doing the division on the result of the sum (so after everything has been added together), not in the method

7

thankks for all the help! i think today was a bad try 2 try and study havnt been sleeping well lol but i got it 2 work using this

let arr = [2,4,6,8]
const newArr = arr.reduce((total, amount, index, array) => {
  total += amount;
  if( index === array.length-1) { 
    return total/array.length;
  }else { 
    return total;
  }
});
console.log(newArr)

i thought return ended a function so using it twice confused me im really bad at this lol is it that return is returning it to the accumulator untill the its went over the whole array and then the final return, returns it to the average variable or am i still getting confused on how this works? sorry for the all the questions.

8

That’s a way but it is not necessary to do that with the double if/else

Do you know of associative property in math?
(a + b) / c = a/c + b/c

You can divide each item before adding it to the total. Or you can make the division outside of the reduce method

1 Like
9

i did cs50 on edex last year before i had an trumatic event which ment i cudnt study for about a year and remember things like big O notation. ill look up associative propertys if theres any other bits of maths which would help write more efficient algorithms I would be intrested in looking at it if its not 2 insanely taxing. (i think thats what your getting at the method above would have a better runtime?)

im enjoying learning this again but its very hard and some days it feels alot harder than others lol im really thankful for your feedback! tomorow ill be going over more uses of reduce to get a better understanding of how it works im getting there i think.