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Working with Hash Keys

4:28 with Jason Seifer

Hashes are composed of keys and values. In this video, we'll learn how to work with hash keys.

Code Samples

Here is the hash we'll be working with:

hash = { "item" => "Bread", "quantity" => 1, "brand" => "Treehouse Bread Company" }
Keys

To find out all of the different keys inside of the hash, we can use the keys method:

hash.keys

This would return an array of the keys in the hash:

["item", "quantity", "brand"]

To check whether or not a hash contains a key, we can use the has_key? method, which returns true or false. It is aliased as member? and key?:

hash.has_key?("brand")      # => true
hash.member?("quantity")    # => true
hash.key?("item")           # => true

The store method will add a key and value pair to a hash:

hash.store("calories", 100)

The hash would then contain the following:

{ "item" => "Bread", "quantity" => 1, "brand" => "Treehouse Bread Company", "calories" => 100 }
Equality

Two hashes are considered equal when they have the same keys and values:

milk = { "item" => "Milk", "quantity" => 1, "brand" => "Treehouse Dairy" }

puts milk == hash     # => true

bread = { "item" => "Bread", "quantity" => 1, "brand" => "Treehouse Bread Company" }

puts hash == bread     # => false

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    Now that we know how to create a hash and the basics of accessing the items inside, 0:00
    we can take a look at how to work with the different identifiers in the hash, 0:06
    the hash keys. 0:10
    Ruby, provides us a bunch of different methods for 0:12
    working with keys inside of a hash which let us write more powerful programs. 0:15
    Let’s take a look at that now using WorkSpaces. 0:21
    Okay, so I've just launched a new Ruby WorkSpace, and 0:25
    here we are down in the console. 0:28
    And let's go ahead and see how we can work with hash keys by themselves. 0:30
    So let's launch IRB by typing in irb. 0:34
    And let's create a hash to work with. 0:38
    This time I'm going to call my hash variable hash. 0:40
    Very creative, I know. 0:44
    And we'll say the item that we want here is "Bread", 0:46
    still using the grocery list metaphor. 0:49
    We'll do a quantity of one and 0:51
    the brand is gonna be "Treehouse Bread Company". 0:55
    All right, so here is our hash. 1:02
    Now if we wanted to see all of the different keys inside of this hash, 1:06
    we could use a method called keys. 1:11
    Say hash and then the dot and the method that we want, which in this case is keys. 1:14
    And that returns an array of all the different keys inside of this hash. 1:21
    Now if we wanted to know if the hash had a certain key in it. 1:28
    And then the method has_key with a question mark, and 1:35
    then we pass in the key that we're curious about. 1:39
    And does it have the key called brand? 1:46
    And at that point Ruby returns true because it does. 1:49
    Now we can also ask it if it has the key brands, though that looks a little bit 1:55
    funny so let's say hash.has_key?("brands") and it does not so 2:00
    it will return true if the hash has the key and false if it does not. 2:04
    Now there a couple of other ways of asking if it has a key. 2:11
    We could say key with a question mark. 2:15
    And that is an alias for hash key and also we can use the word member. 2:19
    Which is another alias for has_key?. 2:25
    Now if we want to access hash keys, let's go ahead and clear the screen here so 2:29
    we've got our hash. 2:34
    Let's go ahead and say hash and 2:36
    a dot and the word fetch and we'll pass it in a key. 2:39
    And that will fetch that given value from the hash with the key that you pass in. 2:45
    And if we look at the hash, it stays unmodified. 2:52
    And this is basically the same as using brackets. 2:55
    It will return the same thing. 3:01
    Now if we ask for something that isn't there, it will say key not found. 3:03
    And the error we get is KeyError. 3:09
    And that's just a little error. 3:12
    Nothing to be concerned with. 3:14
    Don't have to freak out in fact here's a bunch of different ones. 3:17
    Nothing catastrophic happens. 3:22
    I'm gonna clear my screen again. 3:25
    So we've got our hash. 3:27
    Let's make another one called item. 3:29
    And we'll say. 3:31
    This is another hash, with only one key the name is "Milk". 3:35
    Now we could see if two hashes equal each other, 3:40
    which means they have the same keys and values. 3:42
    So if we look at our equality operator, we could say, is the hash equal to this item? 3:46
    And it's not, that's false. 3:52
    But if we create another item, with the same keys and 3:54
    values, that hash will return true. 3:58
    Now another thing that we can do if we wanted to add on to this hash, 4:02
    we could say item and call the method store or 4:05
    let's say we wanted to store the calorie count in here. 4:10
    The first argument is the key and in the second argument is the value. 4:14
    Now if we look at our item, it has a new hash key. 4:19
    Try working with hashes and hash keys now on your own using WorkSpaces. 4:23

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