(Financial application: compare loans with various interest rates)

Question

Write a program that lets the user enter the loan amount and loan period in number of years and displays the monthly and total payments for each interest rate starting from 5% to 8%, with an increment of 1/8.

Sample Run

Loan Amount: 10000

Number of Years: 5

Interest Rate Monthly Payment Total Payment

5.000% 188.71 11322.74

5.125% 189.29 11357.13

5.250% 189.86 11391.59

...

7.875% 202.17 12129.97

8.000% 202.76 12165.84

Answer 1

#!/usr/bin/env python3

def calculate_monthly_payment(principal: float, term_length: float, annual_interest_rate: float) -> float:

"""

Calculate the monthly payment for a loan given the principal, term length, and annual interest rate.

The formula used to calculate the monthly payment is the annuity formula:

M = P[r(1+r)^n]/[(1+r)^n-1]

where:

P = principal

r = monthly interest rate

n = number of payments (term length * 12)

Parameters

----------

loan_amount : float

The amount of money borrowed

term_length : float

The length of the loan in years

annual_interest_rate : float

The annual interest rate of the loan

Returns

-------

float

The monthly payment for the loan

"""

monthly_interest_rate = annual_interest_rate / 12

number_of_payments = term_length * 12

# Monthly payment calculation using the annuity formula

monthly_payment = principal * (monthly_interest_rate * (1 + monthly_interest_rate) ** number_of_payments) / ((1 + monthly_interest_rate) ** number_of_payments - 1)

return monthly_payment

def main() -> int:

try:

principal = input("Enter the loan amount: ")

term_length = input("Enter the term length in years: ")

annual_interest_rate = input("Enter the annual interest rate (as a percentage): ")

principal = float(principal)

term_length = float(term_length)

annual_interest_rate = float(annual_interest_rate) / 100 # Convert percentage to decimal

if principal <= 0:

raise ValueError("The principal must be greater than 0 lol")

if term_length <= 0 or term_length > 100:

raise ValueError("The term length should probably be greater than 0 and no more than 100 years, you crazy?")

if annual_interest_rate <= 0:

raise ValueError("Good luck to whoever is giving you a loan with this rate lol")

if annual_interest_rate >= 1:

raise ValueError("That's a robbery! Who's charging you >= 100% interest?")

monthly_payment = calculate_monthly_payment(principal, term_length, annual_interest_rate)

print(f"The monthly payment for a loan of ${principal} over {term_length} years at an annual interest rate of {annual_interest_rate * 100}% is ${monthly_payment:.2f}")

except ValueError as e:

print(f"Invalid input: {e}")

return 1

return 0

if __name__ == "__main__":

main()