r22395 - gnucash/trunk/src/scm - add futureValue and computeInterestIncrement to fin.scm

[Frank H. Ellenberger]

Author: fell
Date: 2012-09-16 09:17:34 -0400 (Sun, 16 Sep 2012)
New Revision: 22395
Trac: http://svn.gnucash.org/trac/changeset/22395

Modified:
   gnucash/trunk/src/scm/fin.scm
Log:
add futureValue and computeInterestIncrement to fin.scm
as announced by jsled in http://lists.gnucash.org/pipermail/gnucash-user/2005-February/012964.html
additional improving of comments

Modified: gnucash/trunk/src/scm/fin.scm
===================================================================
--- gnucash/trunk/src/scm/fin.scm	2012-09-15 19:03:19 UTC (rev 22394)
+++ gnucash/trunk/src/scm/fin.scm	2012-09-16 13:17:34 UTC (rev 22395)
@@ -15,7 +15,12 @@
 ;; 51 Franklin Street, Fifth Floor    Fax:    +1-617-542-2652
 ;; Boston, MA  02110-1301,  USA       gnu at gnu.org
 
+
+;; Financial functions originally used by the mortgage/loan druid, 
+;; but useful in scheduled transactions
+;; 
 ;; Copyright 2002 Joshua Sled <jsled at asynchronous.org>
+;; Update 2012 Frank H. Elenberger <frank.h.ellenberger at gmail.com>
 ;;
 
 ;; Simple function for testing:
@@ -25,6 +30,7 @@
 ;; positive values to be returned (as gnucash will handle the credit/debit
 ;; appropriately)
 
+;; interest payment amount:
 (define (gnc:ipmt rate per nper pv fv type)
   (* -1 (* rate
            (- 0 (calc-principal pv
@@ -33,6 +39,7 @@
         ))
 )
 
+;; principal payment amount:
 (define (gnc:ppmt rate per nper pv fv type)
   (let* ((pmt (calc-pmt rate nper pv fv type))
          (ipmt (* rate
@@ -42,9 +49,30 @@
            (* -1 ipmt))))
 )
 
+;; payment amount:
 (define (gnc:pmt rate nper pv fv type)
   (* -1 (calc-pmt rate nper pv fv type)))
 
+
+;; 2 functions from http://lists.gnucash.org/pipermail/gnucash-user/2005-February/012964.html
+;; future value of deposits with compound interests:
+(define (gnc:futureValue a r n t)
+    ;; Parameters:
+    ;; a: amount
+    ;; r: interest rate
+    ;; n: frequency per year
+    ;; t: time
+    ;;
+    ;; formula from http://www.riskglossary.com/articles/compounding.htm
+  (* a (expt (+ 1 (/ r n)) (* n t))))
+
+(define (gnc:computeInterestIncrement amount interest periods i)
+  (let ((thisVal (gnc:futureValue amount interest periods i))
+        (prevVal (gnc:futureValue amount interest periods (- i 1))))
+    (- thisVal prevVal)
+  )
+)
+
 ;;;;;
 ;; below: not-exposed/"private" functions, used by the "public" functions
 ;; above.
@@ -105,7 +133,7 @@
 ;; holding true for all calculated numbers. However, this won't fix the first problem if your bank
 ;; can't do proper maths and manual fixing of transactions will still be required.
 
-;; One problem with the rounding procedure in these three functions is that it is always 
+;; FIXME: One problem with the rounding procedure in these three functions is that it is always 
 ;; rounding at the second decimal. This works great with dollars and euros and a lot of major 
 ;; currencies but might well cause issues with other currencies not typically divided in 100. 
 ;; I have not tested anything else than dollars.
@@ -118,7 +146,7 @@
 ;; On the opposite side, if you want the automatic rounding but don't understand how to use
 ;; the cpd_ functions, here is a quick example on how to convert original gnc:Zpmt
 ;; function calls. The typical setup is to use 'rate/yfreq' as the first parameter, so the 
-;; slution is to simply use yfreq for both yfreq and ycomp in the gnc:cpd_Zpmt calls, like this:
+;; solution is to simply use yfreq for both yfreq and ycomp in the gnc:cpd_Zpmt calls, like this:
 ;;                         gnc:pmt( rate  /  yfreq   :nper:pv:fv:type)
 ;; is equivalent to    gnc:cpd_pmt( rate:yfreq:yfreq :nper:pv:fv:type)