用 Python 计算抵押贷款利息

Calculating mortgage interest in Python

提问by BeerHuntor

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)

我目前正在通过 youtube 上的视频教程学习 python，并且遇到了一个我似乎无法理解的公式，因为对我来说没有什么是正确的。练习的基本概念是制作一个抵押计算器，要求用户输入 3 条信息，贷款金额、利率和贷款期限（年）

then it calculates the monthly payments to the user. here is my code:

然后它计算每月支付给用户的费用。这是我的代码：

__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.

一切正常，但它最终抛出的价值是完全错误的……一笔 100 英镑的贷款，一年内的利率为 10%，不应该让我每月支付 0.83 英镑。任何帮助我解决这个等式以帮助我理解的帮助将不胜感激。

回答by Alfista71

Apparently you copied the formula wrong.

显然你复制了错误的公式。

wrong:   * numberOfPayments 

correct: ** numberOfPayments 

note: it appears twice in the formula note: in python, **is "to the power of" operator.

注意：它在公式中出现两次 注意：在python中，**是“to the power of”运算符。

回答by Letlaka Tsotetsi

I also came accross this problem and this is my solution

我也遇到了这个问题，这是我的解决方案

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))

回答by bddmass

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)

我也看了这个 youtube 视频并遇到了同样的问题。我是一个 python 新手，所以请耐心等待。视频中的讲师使用的是 python3，而我使用的是 python2，所以我不确定所有语法是否都相同。但是使用此线程中的一些信息和此链接中的信息：（http://www.wikihow.com/Calculate-Mortgage-Payments）我能够得到答案。（我的计算与在线抵押计算器相符）

#!/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)

回答by C. Ramsey

With the help of examples, this is what I did.

在示例的帮助下，这就是我所做的。

# 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)

回答by Justin B

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.

这对我来说效果很好。不准确，因为贷款计算器通常按天计算，而我很懒，所以我按月计算，但这是我写的一个简单的计算器，它足够准确实用。

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)

回答by kravietz

This mortgagepackage does it nicely:

这个抵押贷款包做得很好：

>>> 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