pythonloopsvariableswhile-loop

i don't know why the terminal keep running and doesn't show any result


this is the exercise that work i on: You have graduated from MIT and now have a great job! You move to the San Francisco Bay Area and decide that you want to start saving to buy a house. As housing prices are very high in the Bay Area, you realize you are going to have to save for several years before you can afford to make the down payment on a house. In Part A, we are going to determine how long it will take you to save enough money to make the down payment given the following assumptions:

  1. Call the cost of your dream home total_cost.
  2. Call the portion of the cost needed for a down payment portion_down_payment. For simplicity, assume that portion_down_payment = 0.25 (25%).
  3. Call the amount that you have saved thus far current_savings. You start with a current savings of $0.
  4. Assume that you invest your current savings wisely, with an annual return of r (in other words, at the end of each month, you receive an additional current_savings*r/12 funds to put into your savings – the 12 is because r is an annual rate). Assume that your investments earn a return of r = 0.04 (4%).
  5. Assume your annual salary is annual_salary.
  6. Assume you are going to dedicate a certain amount of your salary each month to saving for the down payment. Call that portion_saved. This variable should be in decimal form (i.e. 0.1 for 10%).
  7. At the end of each month, your savings will be increased by the return on your investment, plus a percentage of your monthly salary (annual salary / 12). Write a program to calculate how many months it will take you to save up enough money for a down payment. You will want your main variables to be floats, so you should cast user inputs to floats.
    1 Your program should ask the user to enter the following variables:
  8. The starting annual salary (annual_salary)
  9. The portion of salary to be saved (portion_saved)
  10. The cost of your dream home (total_cost) Hints To help you get started, here is a rough outline of the stages you should probably follow in writing your code: ● Retrieve user input. Look at input() if you need help with getting user input. For this problem set, you can assume that users will enter valid input (e.g. they won’t enter a string when you expect an int) ● Initialize some state variables. You should decide what information you need. Be careful about values that represent annual amounts and those that represent monthly amounts. Try different inputs and see how long it takes to save for a down payment. Please make your program print results in the format shown in the test cases below.

Test Case 1

Enter your annual salary: 120000, Enter the percent of your salary to save, as a decimal: .10, Enter the cost of your dream home: 1000000, Number of months: 183

Test Case 2

Enter your annual salary: 80000, Enter the percent of your salary to save, as a decimal: .15, Enter the cost of your dream home: 500000, Number of months: 105, this is my code:



#variables and inputs:
current_savings=0
r=0.04
additional_current_savings=current_savings*r/12
annual_salary=float(input("Enter your annual salary: "))
portion_saved=float(input("Enter the percent of your salary to save, as a decimal: "))
total_cost=float(input("Enter the cost of your dream home: "))
portion_down_payment=0.25*total_cost
monthly_salary=annual_salary / 12
new_total_saving=0
number_of_months=0
total_saving=portion_saved+current_savings+additional_current_savings
#a program to calculate how many months it will take someone to save up enough money for a down payment:
while new_total_saving != portion_down_payment :
 new_total_saving += total_saving
 number_of_months += 1
print(f"Number of months: {number_of_months}")

this is the expected output: Enter your annual salary: 120000 Enter the percent of your salary to save, as a decimal: .10 Enter the cost of your dream home: 1000000 Number of months: 183

i know that my program isn't all encompassing,but it should have solved atleast test case 1. i don't know why the terminal keep running without any result. Thanks in advance.


Solution

  • In the code, the while loop condition was new_total_saving != portion_down_payment. The issue here is that due to the nature of floating-point arithmetic, it's possible that new_total_saving never exactly equals portion_down_payment due to small precision errors. Hence, the loop will run indefinitely because new_total_saving will never be exactly equal to portion_down_payment. So, your terminal seems running because the loop never terminated.

    What you really want according to your problem statement is that your loop should run until new_total_saving becomes greater than or equal to portion_down_payment so that you have at least the down payment money saved. To fix this issue, you should use a less than inequality operand (<).

    Additionally in your program,

    1. Remove unnecessary variables (additional_current_savings, new_total_saving, and total_saving)
    2. Modify the loop condition to check if current_savings is less than portion_down_payment.
    3. Update the calculation of current_savings within the loop to properly account for both investment returns and monthly savings as following:
    while current_savings < portion_down_payment:
        # Update current_savings with investment return
        current_savings += current_savings * r / 12
        
        # Add monthly savings from salary to current_savings
        current_savings += monthly_salary * portion_saved
        
        # Increment the number of months
        number_of_months += 1