mort.py

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##
# 
# Program that will print out a table of factors to derive a
# monthly payment for a mortgage, given the interest rate in %/yr
# and the length of the mortgage in years.  The user divides the
# principal by 1000 and multiplies this by the table entry to get
# the monthly payment.
# 
# Copyright (C) 2002 GDS Software
# 
# This program is free software; you can redistribute it and/or
# modify it under the terms of the GNU General Public License as
# published by the Free Software Foundation; either version 2 of
# the License, or (at your option) any later version.
# 
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
# 
# You should have received a copy of the GNU General Public
# License along with this program; if not, write to the Free
# Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
# MA  02111-1307  USA
# 
# See http://www.gnu.org/licenses/licenses.html for more details.
# 

import math, sys
__version__ = "$Id: mort.py,v 1.4 2002/08/21 12:41:48 donp Exp $"

interest_step  = 0.5
begin_interest = 2.0
end_interest   = 15.0

years = [3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30]

fp = sys.stdout

# Print the header information 
fp.write("Monthly payment per $1000 principal\n\n")
fp.write("                                   Years\n")
fp.write("%/yr     ")
for year in years:
    fp.write("%2d    " % year)
fp.write("\n" + "-" * 6)
fp.write("-" * 6 * len(years) + "\n")

interest = begin_interest
factor_array = []

while interest <= end_interest:
    # Calculate the current row's factors 
    factor_array = []
    for i in xrange(len(years)):
        tmp=math.pow(1+interest/1200.0, -years[i]*12.0);
        factor = 1000.0*(interest/1200.0)/(1.0-tmp);
        factor_array.append(factor)
    # Now print the factors
    fp.write("%5.2f  " % interest)
    for i in xrange(len(years)):
        fmt = "%5.2f "
        if factor_array[i] < 10.0:
            fmt = "%5.3f "
        if factor_array[i] >= 100.0:
            fmt = "%5.1f "
        fp.write(fmt % factor_array[i])
    fp.write("\n");
    interest = interest + interest_step

fp.write("\nExample:  A 10 year loan of $38000 at 8.0% per year will require\n")
fp.write("a payment of 38 * 12.13 = $461.\n\n")

fp.write("Formula:  let i = yearly interest in %\n")
fp.write("              T = time in years\n")
fp.write("              A = (1 + i/1200)^(-T*12)\n")
fp.write("Then factor = 1000 * (i/1200)/(1 - A)\n")