Finding an average after replacing a current value

问题

Not sure why this problem is occurring, but for some reason I'm getting the wrong output. What I'm trying to do is take an existing average of ratings and generate a new one. The case I'm currently solving is when a user who has already been calculated in the average needs their value changed.

For example:

User A rates at 5 stars
User B rates at 3 stars
User C rates at 5 star

Current average stars is: 13/3 or 4.333

Now if user B changes their rating, and I don't want to go cycling through all ratings, I want to just say:

Given that I know there are 3 ratings and the one I'm changing is user C to 3, then I would presume I can do:

13-5 = 8
8 + 3 = 11
11/3 = new average 3.667

var originalRatingVal = originalRating.rating ? originalRating.rating.stars : 0
var currentNumberOfRatings = currentItem.count ? currentItem.count.ratings : 0

var currentAverageTotal = currentNumberOfRatings * overall_rating
var oldNumberOfRatings = currentNumberOfRatings - 1
var newAverage = (currentAverageTotal - originalRatingVal) - oldNumberOfRatings

var newRatingVal = currentRating.rating ? currentRating.rating.stars : 0
var newNumRatings = oldNumberOfRatings + 1;
var oldRatingTotal = newAverage * oldNumberOfRatings;
var newAvgRating = (oldRatingTotal + newRatingVal) / newNumRatings;

This doesn't seem to be working:

If I change user B to 5, expecting newAvgRating to equal 5, it currently equals 5.666666666666667.

Any ideas would be extremely appreciated, thank you!

回答1:

const ratings = [5, 3, 5];
const ratingsTotalled = ratings.reduce((a,b)=>a+b);
const originalAverage = ratingsTotalled/ratings.length;
console.log('original:', originalAverage);

//now, I want to remove a '5' rating
//and replace it with '3'

const alteredTotal = ratingsTotalled - 5 + 3;
const alteredAverage = alteredTotal/ratings.length;

console.log('altered:', alteredAverage);

In other words:

const originalRatings = {
  a: 5,
  b: 3,
  c: 5,
}
const ratings = Object.values(originalRatings);
const ratingsTotalled = ratings.reduce((a,b)=>a+b);
const originalAverage = ratingsTotalled/ratings.length;
console.log('original:', originalAverage);

//now, I want to remove a '5' rating
//and replace it with '3'
const cAlteredRating = 3;
const alteredTotal = ratingsTotalled - originalRatings.c + cAlteredRating;
const alteredAverage = alteredTotal/ratings.length;

console.log('altered:', alteredAverage);

来源：https://stackoverflow.com/questions/56843475/finding-an-average-after-replacing-a-current-value

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javascript

logic

average