Exercise: House Hunting

The only way to get better at a skill is to deliberately practice. (Deliberate practice is a special type of preparation that is specific, purposeful, and mindful of the aspects of a skill where you are weak.) This article is a part of a series introducing programming challenges in the Python programming language to provide opportunities for deliberate practice. Each entry in the series will have two parts, this part introduces the requirements for a program needed to calculate the amount needed for a house down payment. The next will show my solution and walk through the decisions I made while writing it.

House Hunting

You have just accepted a job in the San Francisco Bay area and you want to (eventually) buy a home. Since housing prices are very high in the Bay Area, you want to plan your savings and determine how long you will need to save before you can afford a down payment. To model a few different scenarios, you decide to create a simple program.

Requirements

The assumptions you are currently making are:

1. The value of the dream host should be set when the program starts: total_cost
2. Portion of the cost needed for a down payment will also be constant (`portion_down_payment`) and can be set to 25% of the value of the home ( 0.25 )
3. The amount you have currently saved can be modeled in a single variable ( current_savings )
4. Assuming that you invest your current savings wisely, with annual return of r (in other words, at the end of each month, you receive an additional current_savings*r/12 payment into your savings as interest). Assume that your investments earn a return of r=0.04 (4%).
5. Assume the annual salary is annual salary.
6. Assume you are going to dedicate a certain amount of your salary each month to saving of the down payment. Call that portion_saved.
7. At the end of the month, the savings will increase by the return on your investment plus a percentage of your monthly salary (annual salary / 12).

Your program should ask for the following variables:

1. The starting annual salary: annual_salary
2. The portion of the salary to be saved: portion_saved
3. The cost of the dream home: total_cost

Hints

1. Organize your program into the following sections: input, process, iteration
2. Initialize some state variables to ensure that all values are available at the time you begin iterating

Test Cases

Test Case 1

• Cost of House: $1 million
• Annual Salary: $120,000
• Percent of salary: 0.1
• Number of months: 183

Test Case 2

• Cost of House: $500,000
• Annual Salary: $80,000
• Percent of salary: 0.15
• Number of months: 105