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Ruby Hash Creation

6:35 with Jason Seifer

Let's start working with hashes now. Hashes are created in a couple of different ways. A hash can be instantiated using curly braces or instantiating a new instance of the class.

Code Samples

Here's how to create a hash by instantiating a new instance of the Hash class:

item = Hash.new

Hashes can also be created by using curly braces:

{}

When creating a hash using curly braces, keys and values can also be specified: 

item = { "item" => "Bread", "quantity" => 1 }

Hash keys can be almost any Ruby type. Here's an example of using symbols as hash keys:

item = { :item => "Bread", :quantity => 1 }

Hash keys can also be numbers. If we set the following hash key and value:

item[1] = "Grocery Store"

Our hash would look like this:

{ :item => "Bread", :quantity => 1, 1 => "Grocery Store" }

Once a hash has been instantiated, it is possible to add new hash keys and values by using the name of the hash, brackets containing the new key, an equals sign, and the new value for said key:

item["brand"] = "Treehouse Bread Company"

If you're using symbols as keys, there's a shorthand notation you can use. So if you're defining a hash like this...

item = { :item => "Bread", :quantity => 1 }

...you can move the colons after the symbols, and omit the arrows. This hash will be identical to the one above:

item = { item: "Bread", quantity: 1 }

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    [MUSIC] 0:00
    In Ruby, a hash is similar to an array. 0:04
    Both arrays and hashes allow us to store different things and 0:08
    multiple different kinds of things. 0:12
    Hashes are a little bit different than arrays though. 0:15
    In an array we can only refer to an item based upon its position or 0:18
    index in the array. 0:24
    Inside of a hash, we use an identifier to describe the item that we wanna access. 0:26
    The identifier can be almost any Ruby type, 0:32
    but the most common are strings, numbers, and symbols. 0:35
    Let's take our grocery list example from earlier and 0:41
    say that we wanted to store the number of items we wanted to buy on our grocery list 0:44
    along with the list itself. 0:48
    We could use a hash for that and say we store the item, which would be bread, 0:51
    and the quantity of one. 0:55
    When we refer to the items in a hash, the identifier that we use is called the key. 0:57
    And what we store there is called the value. 1:04
    In this case, the item would be one key, and bread, the value. 1:08
    Quantity is also a key, and one would be the value there. 1:13
    We write hashes similarly to how we write arrays. 1:19
    Let's create a hash now using WorkSpaces. 1:23
    Okay, so I've just launched a new Ruby workspace, and let's go ahead and 1:27
    launch irb and see how we work with hashes. 1:31
    Now with arrays we were creating our grocery list. 1:36
    Now let's concentrate on individual grocery list items. 1:39
    So we'll say the item is a new hash. 1:44
    Now we write that out just like we did with arrays by saying Hash, 1:47
    with a capital H, a dot and new. 1:52
    Now, just like arrays, we also have a shortcut for 1:56
    creating a hash, which is going to be the curly braces. 2:01
    And we have an opening curly brace, and a closing curly brace. 2:05
    Now we can see that Ruby returns this, and that is an empty hash. 2:11
    But empty hashes are no fun, so let's go ahead and create our grocery list item. 2:16
    And we'll say the name is bread, and here we've created our first hash. 2:22
    Now I've done this by separating the curly brace with a space. 2:29
    That is totally optional. 2:34
    Now the way that we do this, in this instance, name here is a string, and 2:37
    that is the key, and bread is also a string, and that is the value. 2:41
    So name is the key, bread is the value. 2:50
    So if we wanted to find out the value at the hash key of name, 2:54
    we would type in the name of the hash, which is item, and then opening brackets 2:59
    just like we accessed arrays, and then the key that we wanna retrieve, 3:04
    which in this case is name, and that will return the value, which is bread. 3:08
    Now we can create hashes with more than one hash key and value. 3:15
    We can say name is bread, and since this is a grocery list, 3:20
    maybe we would want to buy more than one loaf of bread, and 3:25
    we could say, we could do a quantity of 1. 3:29
    And that will return our hash, 3:33
    now it has two keys, one being name, the other being quantity. 3:37
    And values, bread and 1, which is a number. 3:41
    Hash keys can be almost any Ruby type or object, and so could hash values as well. 3:46
    Hash keys, let's go ahead and create one, we'll just say, we'll use a number here, 3:54
    so 1, that could be a grocery store. 3:59
    Doesn't actually mean anything, but if we look back at our hash now, 4:03
    we have added the key of 1 with the value of grocery store. 4:07
    Doesn't quite make sense in this context. 4:12
    Just wanted you to see that you could use a number as a hash key. 4:14
    And let's go ahead and add another key here. 4:21
    And we can add a key to an existing hash by typing the name of the hash, 4:23
    opening the bracket, and then the name of the new key. 4:29
    And we'll say the brand of bread that we want. 4:33
    And then we assign this like any other value, so 4:36
    I'll say Treehouse Bread Company, and that will create a new key in the hash. 4:40
    Let me clear my screen here so we can see it a little better. 4:47
    And the name is bread, quantity of 1, and our miscellaneous key of one, 4:49
    which is a grocery store, and the brand is Treehouse Bread Company. 4:54
    Another way that you might see keys written is with symbols. 4:59
    A symbol is just like a string except it's written with a colon in front of it. 5:04
    And so let's just say the item with the name of bread. 5:11
    Now if we look at our item hash. 5:16
    We have another key here name with the value of bread, 5:18
    and this has a colon in front of it. 5:24
    The only difference between these two is that in Ruby's memory 5:26
    there will only be one symbol called name, 5:31
    whereas each time we use a string there's a new one inside of the memory. 5:35
    And we don't really need to worry about that too much right now but 5:40
    if you see that you will occasionally see 5:43
    hash notation with symbols as hash keys instead of strings or numbers. 5:46
    Now let's go ahead and get one out of there. 5:52
    We could say delete and that will return the item at the given hash key. 5:55
    And now here we go. 5:59
    We've got our hash with the name of bread, and 6:00
    the symbol, also with the name of bread. 6:04
    Now, when we're creating a hash, if we wanted to just use symbols as hash keys, 6:07
    you could do it like this. 6:12
    And in that case we don't have to use the equal sign and the arrow. 6:17
    And if we look at our hash, it will be just the same as if we were using symbols 6:22
    as keys and anything else as the value. 6:26
    Try creating some hashes on your own now using WorkSpaces. 6:30

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