Question

I have an array of Mortgage objects

class Mortgage
  attr_accessor :rate, :deposit

  def initialize(deposit, rate)
    @deposit = deposit
    @rate = rate
  end
end

I’d like to reject any mortgage which has a higher rate than another mortgage with the same deposit.

For example, Imagine the following array of mortgages

Mortgage 1, :rate => 3, :deposit => 20
Mortgage 2, :rate => 5, :deposit => 20
Mortgage 3, :rate => 7, :deposit => 30

The method should reject mortgage 2.

Answer 1

mortgages = [
  Mortgage.new(20, 3),
  Mortgage.new(20, 5),
  Mortgage.new(30, 7)
]

mortgages.group_by(&:deposit).map { |deposit, mortgages| mortgages.min_by(&:rate) }

group_by constructs a hash based off the given method, so in this case it gives:

{
  20 => [
          #<Mortgage:0x24 @rate=3 @deposit=20>,
          #<Mortgage:0x2c @rate=5 @deposit=20>
        ],

  30 => [
          #<Mortgage:0x38 @rate=7 @deposit=30>
        ]
}

Then it’s easy enough to just take the mortgage with the minimum rate from each deposit.