Ruby hash basics

Notes on the Le Wagon Hash & Symbols Burger Hash exercise

Hashes can be confusing at first but once you get your head around them they are relatively straight forward.

For this exercise we want to build a counter that will add up the amount of calories there are in different items of food.

To start with we will store the information in a hash which is a more readable way of storing data and works as a sort of database. We store this in a constant as the data will not change and this is denoted by capital letters.

DISHES_CALORIES = {
  "Hamburger" => 250,
  "Cheese Burger" => 300,
  "Big Mac" => 540,
  "McChicken" => 350,
  "French Fries" => 230,
  "Salad" => 15,
  "Coca Cola" => 150,
  "Sprite" => 150
}

We have assigned this hash to a constant of DISHES_CALORIES. The key is written as a string and the ‘hash rocket’ (=>) shows us which value is associated with each key.

Lets write a method that will take some parameters and return to us the total amount of calories in the dishes we give it.

def basic_calories_counter(burger, side, beverage)
  DISHES_CALORIES[burger] + DISHES_CALORIES[side] + DISHES_CALORIES[beverage]
end

Here, we have defined a method which takes three parameters which we have given the names ‘burger, side and beverage’ although they could just as easily be called ‘x, y, z’. This is simply a placeholder name.

The method contains our calculation or concatination for displaying the total amount of calories. We start by targetting the hash DISHES_CALORIES then giving it each parameter using the []. This is saying look inside the DISHES_CALORIES hash and find the key corresponding with the argument given. We add each item together with the + and this returns the total number of calories. Remember a block will automatically return the last line so there is no need to explicilty write return.

Lets make this more complex by introducing a second hash but this time it contains meals instead.

MEALS = {
  "Happy Meal" => ["Cheese Burger", "French Fries", "Coca Cola"],
  "Best Of Big Mac" => ["Big Mac", "French Fries", "Coca Cola"],
  "Best Of Royal Cheese" => ["McChicken", "Salad", "Sprite"]
}

This hash MEALS is storing meal names as a key and an array of items from the other hash DISHES_CALORIES as the value. We would like to be able to give ‘Happy Meal’ as a parameter and have it return the correct amount of calories in a happy meal.

Start by writing a new method.

def calories_counter(orders)
  counter = 0
  orders.each do |order|
    if DISHES_CALORIES.key?(order)
      counter += DISHES_CALORIES[order]
    else
      counter +=basic_calories_counter(MEALS[order][0], MEALS[order][1], MEALS[order][2])
    end
  end
  return counter
end

Lets go through this step by step.

1. Method name of calories_counter which takes one parameter (orders)

2. Initalize the counter at 0

3. Iterate over the parameter ‘orders’ using .each. We will use order as our placeholder which represents each individual item given in orders.

4. An if statement where we check to see if DISHES_CALORIES contains a key which it found when it iterated over the orders parameter. We do this by calling the ruby method .key? on the hash name and giving it the parameter of order (representing the individual items in orders.) If this returns true it will add and reasign it to our variable counter. The notation DISHES_CALORIES[order] is us saying look inside the hash and give us the value for each order that we want.

5. The else statement is for any keys that are not contained within DISHES_CALORIES. Here we call the method basic_calories_counter and give it the three parameters it requires. The parameters MEALS[order][0] tell it to look inside MEALS and find the order matching the order it has been given. For example it has been given Happy Meal so it will look like this:

def basic_calories_counter(burger, side, beverage)
  DISHES_CALORIES(MEALS[Happy Meal][0] + DISHES_CALORIES(MEALS[Happy Meal][1] + DISHES_CALORIES(MEALS[Happy Meal][2]
end

The method can now see that it must go to MEALS and find the order and then simply select each item at the given indexes 0, 1 and 2 which refer to items 1, 2 and 3. It is able to access the data for each item because DISHES_CALORIES holds that information.

6. We can then return the counter with the total amlunt of calories.