在数组中的多个项目中分配货币/数字

Question

So I have a sort of payment splitter and I'm struggling with floating point operations. I'm using currency.js to handle the maths and have jQuery installed too, though probably won't use it in the example below

I want someone to be able to pay an amount (let's say $50) and spread that amount across all the items in the list, and subtract it.

So say I have a list of prices

prices = [12, 8, 25, 14, 12, 20, 0.5, 8] 
grandTotal = sumArray(prices) //Returns 99.50

Then a customer wants to apply $50 to that invoice, and I want to spread the amount out evenly across all items. So in short I want to do 99.5-50 = 49.5

amountToPay = 50

What I want is to output something like this:

/*This sums to 50 and is what need to be subtracted.
In this case I've added the remainder to the last item, but would like the spread to be as even as possible. In currency format, so two decimal places of precision*/
toSubtract = [6, 4, 12.5, 7, 6, 10, 0.25, 4.25]

/*This sums to 49.5, the amount left over when toSubtract is subtracted from prices*/
newPrices = [6, 4, 12.5, 7, 6, 10, 0.25, 3.75] 

But yeah, really not sure how to even approach distribution. I have attempted to it by calculating the percentage (eg 50/99.5), but it was extremely messy and still had precision & rounding errors everywhere and I was manually catching each one and it was just gross and incomprehensible.

Answer 1

Based on your info, you can do simple math to calculate the partial amounts left.

total = 99.50;
amount = 50.00;
prices = [12, 8, 25, 14, 12, 20, 0.5, 8];
rate = (total - amount) / total;
newPrices = prices.map(e => ((e * rate).toFixed(2)));

["5.97", "3.98", "12.44", "6.96", "5.97", "9.95", "0.25", "3.98"]