Working with decimals in Ruby on Rails 3

Question

I'm trying to calculate the average net price of a product. I have, in my Product model :total_sold and :total_net_revenue. Doing straight division in the method seems to always result in 0. I resorted to using BigDecimal as I figured that was the problem ... but with my latest iteration of the code below, I'm still getting zero when the answer comes out to a decimal.

def avg_price
  BigDecimal(total_sold.to_s) / (BigDecimal(total_net_revenue.to_s) / 100)
end  

Net revenue is in cents, which is why I divide by 100. Can someone point out what I'm doing wrong or should do?

Answer 1

total_net_revenue / total_sold

or

total_net_revenue / total_sold / 100.0

or

total_net_revenue.to_f / total_sold / 100

These three methods give an increasing amount of precision, if you want it. Remember that "average price" is "average price / sale. That's money-per-item so you will want to do the division in that specific order.

Answer 2

First: You are dividing the wrong way.

100 items / $150 = .667 items per dollar

vs.

$150 / 100 items = $1.50 per item

Second: Like other languages, you need to force one of the numbers in the equation to be a decimal so that the result is cast as one as well. Since your revenue is a whole number, that meant all three values were whole numbers, which means you got a whole number as a result. To get a decimal, cast one of them as a floating-point number.

In other words, to get what you need, do this:

price_per_item = (total_net_revenue.to_f / 100) / total_sold