替换当前值后求平均值

Question

不知道为什么会出现这个问题，但由于某种原因，我得到了错误的输出。我正在尝试做的是采用现有的平均收视率并生成一个新的收视率。我目前正在解决的情况是，已经计算出平均值的用户需要更改其值。

例如：

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

当前平均星数为：13/3 或 4.333

现在，如果用户 B 更改了他们的评分，而我不想循环浏览所有评分，我只想说：

鉴于我知道有 3 个评分，而我正在将用户 C 更改为 3，那么我想我可以做到：

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;

这似乎不起作用：

如果我将用户 B 更改为 5，期望 newAvgRating 等于 5，它当前等于 5.666666666666667。

任何想法都将不胜感激，谢谢！

Answer 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);

换句话说：

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);