Hashes

Learning Goals

• Identify the differences between Hashes and Arrays.
• Practice building, accessing, and updating Hashes.

Vocabulary

• Hash
• Symbol
• Key
• Value

Warm-up

Say we have an array named grocery_list that looks like this:

grocery_list = ["milk", "eggs", "eggs", "eggs", "eggs", "eggs", "eggs", "avocado", "avocado", "tortilla", "tortilla", "tortilla", "tortilla", "tortilla", "tortilla", "tortilla", "tortilla", "tortilla"]

Answer these questions with your group:

1. What is problematic about this data being in this array?
2. How would you prefer this data be stored?

Part 1: Student Exploration

With our groups, we will be working through this repository

If you have already completed these exercises, delete your work and let’s do it again! Repetition is key to learning.

While working through these exercises, write down any important information you come across. Take note of:

• What was challenging?
• What patterns did you observe?
• What rules did you discover about how hashes work?
• What syntax did you learn for hashes?
• What questions came up while you were working and how did you get those answered?
• What questions do you still have?
• Anything else that seems important.

Resources to use while working through the exercises:

• Your breakout room mates!
• Google (ex: Ruby how to make a new hash with a default value)
• Ruby Docs
• Intro To Hashes

Part 2: Discussion

Take a minute to look through your notes about Hashes. Add your key takeaways (or AHA! moments) to page 1 of this Jamboard

Take a minute to look through the questions you still have about Hashes. Add them to page 2 of the same Jamboard.

Discussion Questions

When does it make sense to use a Hash? (As opposed to an array?)

Hashes (also called Dictionaries in some languages) are great at keeping track of quantities. They are great when your data can be grouped into key/value pairs.

How should we name our Hashes?

Hashes represent a collection of data. It is usually appropriate to call the hash the plural form of what that data represents.

Ex:

  toppings: {
      pepperoni: 5,
      sausage: 3,
      pineapple: 500
 }

  pets: {
      cats: 6,
      dogs: 9
 }

What data type can a key be?

Keys of a hash can be any data type. Often, we see them as symbols. A symbol is an immutable data-type in Ruby. They will start with a : (colon). Keys can also be seen commonly as strings, however they can be integers as well.

For the following questions, let’s refer to this code snippet:

  grocery_items = {
        eggs: 4,
        bread: 2,
        milk: 3,
        cookies: 4  
  }

How would we find out how many cookies we have?

  grocery_items[:cookies]

We call the name of the hash (the variable we stored the hash in), and then use bracket notation to query the key we want. The return value of the above expression will be the value of the key, so in this case: 4.

How would we add a new key of apples set to a value of 9?

    grocery_items[:apples] = 9

We call the name of the hash, then use bracket notation to denote the key, then use the assignment operator = to set the value of our key to 9.

Practice

Work with your group to write code to complete the following:

1. Create a Hash called office_supplies.
2. Add the key staples with a value of 9000.
3. Add the key paper with a value of 45050.
4. Add the key chairs with a value of 5.
5. Add the key desks with a value of 2.
6. Access the value of staples.
7. Increase the value of paper by 2000. (With code, not manually changing the value in the hash.)
8. Get a list of all your keys. (What data type does the word ‘list’ usually indicate?)
9. Get a list of all the values of your hash.
10. Write code to list out all your keys and values in a string like this: "Office Supplies: 9000 staples, 45050 paper, 5 chairs, 2 desks."

Check Your Understanding

How would you answer these questions if you were asked about this in an interview?

1. What is a hash?
2. What situations would you use a hash for?
3. What are the similarities and differences between Arrays and Hashes?
4. What are a few methods you could call on a hash and what do they do?