logenkain/mortgage-calculator
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Add largest affordable loan

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Logen Kain 2020-10-28 16:41:15 -04:00
parent 73b7574190
commit 4280ae7585
1 changed files with 35 additions and 16 deletions
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mortgage-calculator.lisp
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@ -1,16 +1,6 @@
;;;; Mortgage Calculators ;;;; Mortgage Calculators
#| #|
loan amount (p)
interest rate (r)
years (t)
payments per year (n)
loan type: fixed-rate
Payment = P x (r/n) * [1+(r/n)^n * t]/(1+r/n)^n *t - 1
or
P = payment P = payment
PV = Present Value PV = Present Value
r = rate per period r = rate per period
@ -18,16 +8,45 @@ n = number of periods
P = (r(PV)) / (1-(1+r)^-n) P = (r(PV)) / (1-(1+r)^-n)
Okay, but I want to find PV instead
Guess I need to remember algebra
P = (r(PV)) / (1-(1+r)^-n)
P * (1-(1+r)^-n) = r * PV
(P * (1-(1+r)^-n) / r = PV
monthly-payments conversion
(find-payments loan-total (/ rate 12) (* number-of-payments 12)))
find-payments = loan-total
|# |#
(defun payments (loan-total rate number-of-payments) (defun find-payments (loan-total rate number-of-payments)
"Base loan calculation"
(/ (* rate loan-total) (/ (* rate loan-total)
(- 1 (expt (+ 1 rate) (* number-of-payments -1))))) (- 1 (expt (+ 1 rate) (* number-of-payments -1)))))
(defun monthly-payments(loan-total rate number-of-payments)
"Number of payments in years" (defun find-monthly-payments(loan-total rate number-of-payments)
(payments loan-total (/ rate 12) (* number-of-payments 12))) "Modify loan calculation to show monthly payments"
(defun payment-total-per-year (loan-total rate number-of-payments) (find-payments loan-total (/ rate 12) (* number-of-payments 12)))
(* (payments loan-total (/ rate 12) (* number-of-payments 12)) 12))
(defun find-montly-payment-total-per-year (loan-total rate number-of-payments)
"Modify loan calculation to show total montly payments per year"
(* (find-payments loan-total (/ rate 12) (* number-of-payments 12)) 12))
;; fixme: add function to take monthly-payments rate num-of-payments
(defun find-biggest-loan (payments rate number-of-payments)
"Base loan calculation"
(/ (* payments
(- 1 (expt (+ 1 rate)
(* -1 number-of-payments))))
rate))
(defun find-biggest-loan-monthly (payments rate number-of-payments)
(find-biggest-loan payments (/ rate 12) (* number-of-payments 12)))