- Iterate over a nested hash
In Ruby, it's possible to use enumerable methods to iterate over key-value pairs in hashes. For example:
jon_snow = {
name: "Jon",
email: "[email protected]"
}
jon_snow.each do |key, value|
puts "Key: #{key}"
puts "Value: #{value}"
end
# Key: name
# Value: Jon
# Key: email
# Value: [email protected]What happens when we want to iterate over a nested hash like the one below? Let's iterate over our nested hash one level at a time; iterating over the first level of our hash would look like this:
contacts = {
"Jon Snow" => {
name: "Jon",
email: "[email protected]",
favorite_ice_cream_flavors: ["chocolate", "vanilla", "mint chip"],
knows: nil
},
"Freddie Mercury" => {
name: "Freddie",
email: "[email protected]",
favorite_ice_cream_flavors: ["strawberry", "cookie dough", "mint chip"]
}
}
contacts.each do |person, data|
puts "#{person}: #{data}"
endThis should return:
Jon Snow: {
:name=>"Jon",
:email=>"[email protected]",
:favorite_ice_cream_flavors=>["chocolate", "vanilla", "mint chip"],
:knows=>nil
}
Freddie Mercury: {
:name=>"Freddie",
:email=>"[email protected]",
:favorite_ice_cream_flavors=>["strawberry", "cookie dough", "mint chip"]
}On the first level, the keys are our contacts' names, "Jon Snow" and "Freddie Mercury", and our values are the hashes that contain a series of key/value pairs describing them.
Let's iterate over the second level of our contacts hash. In order to access
the key/value pairs of the second tier (i.e. the name, email, and other data
about each contact), we need to iterate down into that level. So, we pick up
where we left off with the previous iteration and we keep going:
contacts.each do |person, data|
#at this level, "person" is Jon Snow or Freddie Mercury and "data" is a hash of
#key/value pairs to iterate over the "data" hash, we can use the following line:
data.each do |attribute, value|
puts "#{attribute}: #{value}"
end
endThat should output the following:
name: Jon
email: [email protected]
favorite_ice_cream_flavors: ["chocolate", "vanilla", "mint chip"]
knows:
name: Freddie
email: [email protected]
favorite_ice_cream_flavors: ["strawberry", "cookie dough", "mint chip"]Let's take it one step further and print out just the favorite ice cream flavors. Once again, we'll need to iterate down into that level of the hash, then we can access the favorite ice cream array and print out the flavors:
contacts.each do |person, data|
#at this level, "person" is Jon Snow or Freddie and "data" is a hash of
#key/value pairs to iterate over the "data" hash, we can use the following
#line:
data.each do |attribute, value|
#at this level, "attribute" describes the key of :name, :email,
#:favorite_ice_cream_flavors, or :knows we need to first check and see if
#the key is :favorite_ice_cream_flavors, if it is, that means the VALUE is
#an array that we can iterate over to print out each element
if attribute == :favorite_ice_cream_flavors
value.each do |flavor|
# here, each index element in an ice cream flavor string
puts "#{flavor}"
end
end
end
endThis should output:
chocolate
vanilla
mint chip
strawberry
cookie dough
mint chipBeing able to access data from a nested hash like this gives us a lot of ways to work with this complex data structure and derive insights. What if instead of printing out the favorite ice creams of our contacts, we wanted to collect their email addresses in an array? Well, we could do something like this:
emails = []
contacts.each do |person, data|
data.each do |attribute, value|
if attribute == :email
emails << value
end
end
end
emailsFork and clone this exercise to code along!
Let's do an exercise to get some practice iterating through nested arrays.
You'll be coding your solution in contacts.rb. You'll be manipulating the hash
that is returned by the #contacts method:
def contacts
{
"Jon Snow" => {
name: "Jon",
email: "[email protected]",
favorite_ice_cream_flavors: ["chocolate", "vanilla"]
},
"Freddie Mercury" => {
name: "Freddie",
email: "[email protected]",
favorite_ice_cream_flavors: ["strawberry", "cookie dough", "mint chip"]
}
}
endYour good buddy Freddie 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 Freddie's
favorite ice cream flavors.
There are at least two ways you can accomplish this, and for this code along, we'll work with the second way.
-
You can directly iterate over the hash that is the value of the
"Freddie Mercury"key by calling an enumerator method incontacts["Freddie Mercury"]. -
You can set a iterate through the hash and check for
Freddie Mercuryonly; 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.
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 (like contact_details_hash below).
Add this code inside the #remove_strawberry method:
contacts.each do |person, contact_details_hash|
binding.pry
endWe can enter the Pry session in one of two ways: by running learn test or by
running ruby contacts.rb. We'll use learn test.
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=>"[email protected]", :favorite_ice_cream_flavors=>["chocolate", "vanilla"]}Excellent! They do!
Type exit while in Pry to continue. The binding.pry breakpoint 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
contact_details_hash 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.
Update your code to match the following:
def remove_strawberry(contacts)
contacts.each do |person, contact_details_hash|
if person == "Freddie Mercury"
contact_details_hash.each do |attribute, data|
binding.pry
end
end
end
endAgain, let's jump into our binding.pry using learn test. We can verify that
we've found the record for Freddie Mercury by checking the values of our
variables:
attribute
# => :name
data
# => "Freddie"Remember, if you get stuck and can't enter text in Pry, hit
qto continue!
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-program or !!! at any time to exit out of pry entirely.
Update your code to match the following:
def remove_strawberry(contacts)
contacts.each do |person, contact_details_hash|
if person == "Freddie Mercury"
contact_details_hash.each do |attribute, data|
if attribute == :favorite_ice_cream_flavors
binding.pry
end
end
end
end
endThis 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 Freddie's favorite flavors.
Lastly, we will use the #delete_if array method to iterate through the ice
cream array and remove any element that matches "strawberry". Recall that data
is the array containing Freddie'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 element if the block returns true. Learn
more about #delete_if in the ruby docs..
The full method should now be:
def remove_strawberry(contacts)
contacts.each do |person, contact_details_hash|
if person == "Freddie 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
endCongrats! You made it. Test that your method works by running ruby contacts.rb
in the terminal. It should output the hash without strawberry ice cream. Also,
be sure to run the specs to make sure they pass.
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