Question

Say I have the following class:

class Cashier

  def purchase(amount)
    (@purchases ||= []) << amount
  end


  def total_cash
    (@purchases || []).inject(0) {|sum,amount| sum + amount}
  end

end

This is for learning purposes only, please ignore how unrealistic this may be.

Now in general, the total_cash could be an expensive call to loop through all the items.

I want to know how I can call .inject ONLY if the @purchases variable is dirty i.e. there was something modified.

How would my class be modified to do this?

Answer 1

The simplest approach would be to maintain another variable to indicate whether or not @purchases is dirty. For example:

class Cashier

  def initialize(*args)
    # init @purchases and @total_cash
    @is_purchases_dirty = false
  end

  def purchase(amount)
    (@purchases ||= []) << amount
    @is_purchases_dirty = true
  end


  def total_cash
    return @total_cash unless @is_purchases_dirty
    @is_purchases_dirty = false
    @total_cash = (@purchases || []).inject(0) {|sum,amount| sum + amount}
    return @total_cash
  end

end

A cleaner/simpler approach may be to calculate @total_cash each time the setter is called for purchases. However, this means that you need to always use the setter, even within your class. It also means that you will be “hiding” an expensive operation inside of a setter method. You can decide which one you like better.

class Cashier

  def purchase(amount)
    (@purchases ||= []) << amount
    @total_cash = (@purchases || []).inject(0) {|sum,amount| sum + amount}
  end


  def total_cash
    @total_cash
  end

end

I would also recommend against your naming scheme for an expensive operation. I would rename total_cash to something like calc_total_cash in order to tell users of your API that this is a relatively expensive call as opposed to a simple getter/setter.