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Savings Account
Savings Account

Savings Account

Learning Exercise

Introduction

Modules

So far all the exercises you have seen have used classes. Sometimes we do not need the overhead of creating an object with state, so instead we use module.

A module is very similar to a class (in fact, Module is Class' parent in the object hierarchy) - the main difference being that rather than using instance methods, we use class methods. Class methods start with self. and are directly called on a module.

For example:

module Speaker
  def self.echo(something)
    "#{something} ... #{something}"
  end
end

Speaker.echo("Hello")   #=> "Hello ... Hello"

Loops

There are several ways to write loops in Ruby, but as we tend to use enumeration rather than looping in general, the most commonly seen loop perhaps is the while loop:

counter = 0

while counter < 5
  counter += 1
end

You can also use its sibling until

counter = 0

until counter == 5
  counter += 1
end

Instructions

In this exercise you will be working with savings accounts. Each year, the balance of a savings account is updated based on the interest rate. The interest rate the bank gives depends on the amount of money in the accounts (its balance):

  • 0.5% for a non-negative balance less than 1000 dollars.
  • 1.621% for a positive balance greater than or equal to 1000 dollars and less than 5000 dollars.
  • 2.475% for a positive balance greater than or equal to 5000 dollars.
  • 3.213% for a negative balance (results in negative interest).

You have three tasks, each of which will deal with the balance and its interest rate.

1. Calculate the interest rate

Implement the SavingsAccount.interest_rate method to calculate the interest rate based on the specified balance:

SavingsAccount.interest_rate(200.75)
#=> 0.5

Note that the value returned is an instance of Float.

2. Calculate the annual balance update

Implement the SavingsAccount.annual_balance_update method to calculate the annual balance update, taking into account the interest rate:

SavingsAccount.annual_balance_update(200.75)
#=> 201.75375

Note that the value returned is an instance of Float.

3. Calculate the years before reaching the desired balance

Implement the SavingsAccount.years_before_desired_balance method to calculate the minimum number of years required to reach the desired balance:

SavingsAccount.years_before_desired_balance(200.75, 214.88)
#=> 14

Note that the value returned is an instance of Integer.

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