lib/exonio/financial.rb
def ipmt(rate, per, nper, pv, fv = 0, end_or_beginning = 0)
      pmt = self.pmt(rate, nper, pv, fv, end_or_beginning)
      fv = self.fv(rate, (per - 1), pmt, pv, end_or_beginning) * rate
      temp = end_or_beginning == 1 ? fv / (1 + rate) : fv

      (per == 1 && end_or_beginning == 1) ? 0.0 : temp
    end
def ipmt(rate, per, nper, pv, fv = 0, end_or_beginning = 0)
      pmt = self.pmt(rate, nper, pv, fv, end_or_beginning)
      fv = self.fv(rate, (per - 1), pmt, pv, end_or_beginning) * rate
      temp = end_or_beginning == 1 ? fv / (1 + rate) : fv

      (per == 1 && end_or_beginning == 1) ? 0.0 : temp
    end
Calculates the payment on interest for an investment based on
 constant-amount periodic payments and a constant interest rate.

 @param rate [Float] The interest rate as decimal (not per cent) per period
 @param per [Integer] The amortization period, in terms of number of periods
 @param nper [Integer] The number of payments to be made
 @param pv [Float] The present value
 @param fv [Float] The future value remaining after the final payment has been made
 @param end_or_begining [Integer] Whether payments are due at the end (0) or
   beggining (1) of each period

 @return [Float]

 @example
   Exonio.ipmt(0.075 / 12, 8, 12 * 2, 5000) # ==> -22.612926783996798