fixed-rent-calculator.py

# calculate and test continuous rent when taking money, also calculate interest rate on financial market
# with help of chatgpt
# Modifying the code to account for a 4% annual increase in the annuity, 
# distributed as a monthly increase, and testing the outcome

def find_monthly_annuity_with_monthly_increase(starting_amount, monthly_interest_rate, total_months, annual_increase_rate):
    lower_bound = 0
    upper_bound = starting_amount  # Increased upper bound to include all possibilities
    tolerance = 0.01  # Tolerance limit for calculation accuracy
    monthly_increase_rate = (1 + annual_increase_rate) ** (1/12) - 1  # Calculating the equivalent monthly increase rate

    while lower_bound <= upper_bound:
        tested_annuity = (lower_bound + upper_bound) / 2
        remaining_balance = starting_amount
        current_annuity = tested_annuity

        for month in range(total_months):
            remaining_balance -= current_annuity
            remaining_balance *= (1 + monthly_interest_rate)
            current_annuity *= (1 + monthly_increase_rate)  # Monthly increase in annuity

        if abs(remaining_balance) < tolerance:
            return tested_annuity  # Found the correct initial monthly annuity
        elif remaining_balance > 0:
            lower_bound = tested_annuity + tolerance  # Increase lower bound
        else:
            upper_bound = tested_annuity - tolerance  # Decrease upper bound

    return tested_annuity  # Returning the best found annuity

# Recalculating the monthly annuity with a monthly distributed 4% annual increase
monthly_annuity_with_increase = find_monthly_annuity_with_monthly_increase(starting_amount=1000000, monthly_interest_rate=0.08 / 12, total_months=10 * 12, annual_increase_rate=0.04)

# Testing the adjusted annuity payment
remaining_balance = 1000000  # Starting amount
current_annuity = monthly_annuity_with_increase

for month in range(10 * 12):
    remaining_balance -= current_annuity
    remaining_balance *= (1 + 0.08 / 12)
    current_annuity *= (1 + (1 + 0.04) ** (1/12) - 1)  # Monthly increase in annuity

monthly_annuity_with_increase, remaining_balance

# Result
# (10117.982961110774, 1.9948362214936544)