Code Along: Manipulating Nested Hashes

Objectives

Iterate through a nested hash
Modify the correct element in a nested hash

Why Nested Hashes Matter

So much of what we do in programming involves storing data in hashes. Often the hashes that we will encounter will have more than one level. As we get into the web, this will become abundantly clear. To build programs in the future, we'll absolutely need to get comfortable working with hashes. Let's get started!

Code Along Exercise

Fork and clone this lab. You'll be coding your solution in lib/contacts.rb. You'll be manipulating the following Hash:

contacts = {
  "Jon Snow" => {
    name: "Jon",
    email: "jon_snow@thewall.we",
    favorite_ice_cream_flavors: ["chocolate", "vanilla"]
  },
  "Freddy Mercury" => {
    name: "Freddy",
    email: "freddy@mercury.com",
    favorite_ice_cream_flavors: ["strawberry", "cookie dough", "mint chip"]
  }
}

Your good buddy Freddy Mercury has recently developed a strawberry allergy! You need to delete "strawberry" from his list of favorite ice cream flavors in the remove_strawberry method.

Iterate over the contacts hash and when you reach the key :favorite_ice_cream_flavors, remove "strawberry" from the Array of Freddy's favorite ice cream flavors.

There are at least two ways you can accomplish this, and for this codealong, we'll work with the second way.

You can directly iterate over the hash that is the value of the "Freddy Mercury" key by calling an enumerator method in contacts["Freddy Mercury"].
You can set a conditional to iterate through the hash for Freddy Mercury only; when you reach the appropriate level, check to see if the key == ("is equal to") :favorite_ice_cream_flavors. If it is, check to see if the array of flavors contains "strawberry". If it does, then delete it from the Array.

Step 1: Iterate over the first level

Inside the remove_strawberry method, let's take our first dive into the contacts Hash. Then we'll use binding.pry to see where we are.

We are going to first iterate over the top level of the Hash where the keys should be the person and the values should be a Hash of details about the person.

Note on variable naming: This process will be remarkably easier if you name your variables to accurately reflect the data they represent. For now, when the value we're iterating over is another hash, we will explicitly add a _hash to the end of the variable name (E.G. contact_details_hash below).

contacts.each do |person, contact_details_hash|
  binding.pry
end

We can enter the pry in one of two ways: by running learn test or by running ruby lib/contacts.rb. We'll use learn test.

Let's run learn test in the terminal and, at the pry prompt, check that our defined variables (person and contact_details_hash) match our expectations.

> person
=> "Jon Snow"

> contact_details_hash
=> {:name=>"Jon", :email=>"jon_snow@thewall.we", :favorite_ice_cream_flavors=>["chocolate", "vanilla"]}

Excellent! They do!

Type exit while in pry to continue. The pry should trigger a second time because we have two contacts. You can verify that we're in the second loop through our hash by checking the values of person and data at the pry prompt.

Typing exit now will end the loop and exit pry since we've finished iterating through our contacts. It will also display the results of the test, which we haven't passed just yet.

Step 2. Iterate over the second level

contacts.each do |person, contact_details_hash|
  if person == "Freddy Mercury"
    contact_details_hash.each do |attribute, data|
      binding.pry
    end
  end
end

Again, let's jump into our binding.pry using learn test. We can verify that we've found the record for Freddy Mercury by checking the values of our variables:

> attribute
=> :name

> data
=> "Freddy"

Before we move on, you will need to exit pry again so you can see the results of the new code we'll be writing in Step 3. We are now inside the loop through the attributes. Because there are three of them, we will need to run exit three times to finish the loop and exit pry. Alternatively, you can run exit! or !!! at any time to exit out of pry entirely.

Step 3. Locate the element we're looking for

contacts.each do |person, contact_details_hash|
  if person == "Freddy Mercury"
    contact_details_hash.each do |attribute, data|
      if attribute == :favorite_ice_cream_flavors
        binding.pry
      end
    end
  end
end

This time we are still iterating through the attributes but we've added a conditional so the pry will only hit when the attribute is equal to :favorite_ice_cream_flavors. If we check the value of data in our binding, we should see the array containing Freddy's favorite flavors.

Step 4. Update the hash

Lastly, we will use delete_if to iterate through the ice cream array and remove any element that matches "strawberry". Recall that data is the array containing Freddy's favorite ice cream flavors. delete_if will iterate through the array, check each element to see if it is equal to "strawberry", and delete the key/value pair if the block returns true. Learn more about it in the ruby docs..

contacts.each do |person, contact_details_hash|
  if person == "Freddy Mercury"
    contact_details_hash.each do |attribute, data|
      if attribute == :favorite_ice_cream_flavors
        data.delete_if {|ice_cream| ice_cream == "strawberry"}
      end
    end
  end
end

The full method should now be:

def remove_strawberry(contacts)
  contacts.each do |person, contact_details_hash|
    if person == "Freddy Mercury"
      contact_details_hash.each do |attribute, data|
        if attribute == :favorite_ice_cream_flavors
          data.delete_if {|ice_cream| ice_cream == "strawberry"}
        end
      end
    end
  end
end

Congrats! You made it. Test that your method works by running ruby bin/contacts in the terminal. It should output the hash without strawberry ice cream. Also, be sure to run the specs to make sure they pass.

sydeast/nested-hash-iteration-code-along