I am currently learning python through a video tutorial on youtube, and have come up against a formula I cannot seem to grasp, as nothing looks right to me. The basic concept of the excersise is to make a mortgage calculator that asks the user to input 3 pieces of information, Loan Amount, Interest Rate, and Loan Term (years)
then it calculates the monthly payments to the user. here is my code:
__author__ = 'Rick'
# This program calculates monthly repayments on an interest rate loan/mortgage.
loanAmount = input("How much do you want to borrow? \n")
interestRate = input("What is the interest rate on your loan? \n")
repaymentLength = input("How many years to repay your loan? \n")
#converting the string input variables to float
loanAmount = float(loanAmount)
interestRate = float(interestRate)
repaymentLength = float(repaymentLength)
#working out the interest rate to a decimal number
interestCalculation = interestRate / 100
print(interestRate)
print(interestCalculation)
#working out the number of payments over the course of the loan period.
numberOfPayments = repaymentLength*12
#Formula
#M = L[i(1+i)n] / [(1+i)n-1]
# * M = Monthly Payment (what were trying to find out)
# * L = Loan Amount (loanAmount)
# * I = Interest Rate (for an interest rate of 5%, i = 0.05 (interestCalculation)
# * N = Number of Payments (repaymentLength)
monthlyRepaymentCost = loanAmount * interestCalculation * (1+interestCalculation) * numberOfPayments / ((1+interestCalculation) * numberOfPayments - 1)
#THIS IS FROM ANOTHER BIT OF CODE THAT IS SUPPOSE TO BE RIGHT BUT ISNT---
# repaymentCost = loanAmount * interestRate * (1+ interestRate) * numberOfPayments / ((1 + interestRate) * numberOfPayments -1)
#working out the total cost of the repayment over the full term of the loan
totalCharge = (monthlyRepaymentCost * numberOfPayments) - loanAmount
print("You want to borrow £" + str(loanAmount) + " over " + str(repaymentLength) + " years, with an interest rate of " + str(interestRate) + "%!")
print("Your monthly repayment will be £" + str(monthlyRepaymentCost))
print("Your monthly repayment will be £%.2f " % monthlyRepaymentCost)
print("The total charge on this loan will be £%.2f !" % totalCharge)
Everything works, but the value it throws out at the end is completely wrong... a £100 loan with an interest rate of 10% over 1 year shouldn't be making me pay £0.83 per month. Any help in getting my head around this equation to help me understand would be greatly appreciated.
With the help of examples, this is what I did.
# Formula for mortgage calculator
# M = L(I(1 + I)**N) / ((1 + I)**N - 1)
# M = Monthly Payment, L = Loan, I = Interest, N = Number of payments, ** = exponent
# Declares and asks for user to input loan amount. Then converts to float
loanAmount = input('Enter loan amount \n')
loanAmount = float(loanAmount)
# Declares and asks user to input number of payments in years. Then converts to float. Years * 12 to get
# total number of months
years = input('How many years will you have the loan? \n')
years = float(years) * 12
# Declares and asks user to input interest rate. Then converts to float and input interest rate is /100/12
interestRate = input('Enter Interest Rate \n')
interestRate = float(interestRate) / 100 / 12
# Formula to calculate monthly payments
mortgagePayment = loanAmount * (interestRate * (1 + interestRate)
** years) / ((1 + interestRate) ** years - 1)
# Prints monthly payment on next line and reformat the string to a float using 2 decimal places
print("The monthly mortgage payment is\n (%.2f) " % mortgagePayment)
I also watched this youtube video and had the same problem. I'm a python newbie so bear with me. The instructors in the video were using python3 and I am using python2 so I am not sure if all the syntax are the same. But using some information found in this thread and info from this link: (http://www.wikihow.com/Calculate-Mortgage-Payments) I was able to get the answer. (My calculations matched an online mortgage calculator)
#!/usr/bin/env python
# The formula I used:
M = L * ((I * ((1+I) ** n)) / ((1+I) ** n - 1))
# My code (probably not very eloquent but it worked)
# monthly payment
M = None
# loan_amount
L = None
# interest rate
I = None
# number of payments
n = None
L = raw_input("Enter loan amount: ")
L = float(L)
print(L)
I = raw_input("Enter interest rate: ")
I = float(I)/100/12
print(I)
n = raw_input("Enter number of payments: ")
n = float(n)
print(n)
M = L * ((I * ((1+I) ** n)) / ((1+I) ** n - 1))
M = str(M)
print("\n")
print("Monthly payment is " + M)
Apparently you copied the formula wrong.
wrong: * numberOfPayments
correct: ** numberOfPayments
note: it appears twice in the formula
note: in python, ** is "to the power of" operator.
This worked pretty well for me. Not exact, as loan calculators usually go by days, and I'm lazy so I go by months but here's a simple one I wrote that is accurate enough to be practical.
L = input("How much will you be borrowing? ")
L = float(L)
print(L)
N = input("How many years will you be paying this loan off? ")
N = float(N) *12
print(N)
I = input("What is the interest in percents that you will be paying? Ex, 10% = 10, 5% = 5, etc. ")
I = float(I)/100
print(I)
M = (L/N) + I*(L/N)
float(M)
print("Your monthly payment will be: ")
print(M)
This mortgage package does it nicely:
>>> import mortgage
>>> m=mortgage.Mortgage(interest=0.0375, amount=350000, months=360)
>>> mortgage.print_summary(m)
Rate: 0.037500
Month Growth: 1.003125
APY: 0.038151
Payoff Years: 30
Payoff Months: 360
Amount: 350000.00
Monthly Payment: 1620.91
Annual Payment: 19450.92
Total Payout: 583527.60
I also came accross this problem and this is my solution
loan = input('Enter Loan Amount: ')
loan = float(loan)
numberOfPayments = input('Enter Loan payments in years: ')
numberOfPayments = float(numberOfPayments) * 12
interest = input('Annuel interest Rate: ')
interest = float(interest)/100/12
monthlyPayments = loan * (interest * (1 + interest) ** numberOfPayments) / ((1 + interest) ** numberOfPayments - 1)
print (round(monthlyPayments))
来源：https://stackoverflow.com/questions/29804843/calculating-mortgage-interest-in-python