Question

I have a standard formula for calculating loan amortization schedule.

Here is the formula:

(Px(i/12))/(1-(1+i/12)^-n)

Here is what I have in ruby:

p = BigDecimal('1000.0')
n = BigDecimal('12')
i = BigDecimal('3')

m = (p * (i/12))/(1-(1+i/12) ** -n)

I’m getting a following error: in `**’: wrong argument type BigDecimal (expected Fixnum) (TypeError)

I’m having hard time trying to play with Fixnun, Float and BigDecimal in ruby.

Answer 1

n must be an integer per definition if you want to use ** aka BigDecimal#power(n)

PS: I just looked up your formula on wikipedia. Since n is the number of payments, it will be an integer by nature so just use a Fixnum for n – you won’t get any troubles 🙂