查找替换当前值后的平均值

Question

我不确定为什么会发生这个问题，但由于某种原因，我得到了错误的输出。我想要做的是取一个现有的评分平均值，并生成一个新的评分。我目前正在解决的情况是，已经计算了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

目前的平均星数是: 13/3或4.333

现在，如果用户B更改了他们的评级，而我不想遍历所有的评级，我只想说：

假设我知道有3个评级，并且我要将用户C更改为3，那么我假设我可以这样做：

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，那么它当前等于5.666666666666667。

任何想法都将非常感谢，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);

​