In this section you will learn more about the functions that can be used to access data in collections, specifically in maps. We will also introduce the following functions for the first time:

get, get-in

get values from a collection based on a key

Remember: If you want to use a collection you define later on in the page then give that collection aname using the function def. Then you can work with that collection just by using its name.

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Collections as maps

We have seen in previous sections how maps can be used to create a much more involved structure to your information, allowing you to represent much more complex things.

Here is an example of a more detailed collection, where some of the values in the map are maps themselves. So instead of {key value} we have {key {key value}} or even {key (key {key value})}.

In this example we model someones very basic Star Wars collection.

(def starwars-characters
  {:jedi   ["Luke Skywalker"
            "Rey"]
   :droids ["R2D2"
            "BB-8"]})

starwars-characters

Finding information in maps

Using the get function we can pull out more specific values from the map. We give the get function the map we want to extract a value from and the key that is paired with the value.

(get starwars-characters :jedi)

Get the droids from the starwars-characters collection

()

A more detailed collection

We want to add ships to our collection, so we have refactored our map to have :characters

(def starwars-collection
  {:characters {:jedi   ["Luke Skywalker"
                         "Rey"]
                :droids ["R2D2"
                         "BB-8"]}
   :ships {:rebels ["Millenium Falcon"]}})

starwars-collection

Hint: You may have noticed that the values are actually a vector of values. We can use any collection as our value.

Getting values from nested maps

In the previous example the key we use to get values from a map was at the top level, ie. it is not nested in another map.

If we want to get a nested value we can use a variation of the get function called get-in. The get-in function takes the map and a vector of keys.

(get-in map [keys])

Get all the jedi characters

()

You could use the get function twice: (get (get starwars-collection :characters) :jedi). Its more common to use the get-in function when there are nested maps though.

Get all the ships that belong to the rebels

()

Once you have completed the exercises, take a look at the suggested examples

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A little short-cut with keywords

In these most recent examples, the keys used in the map have been defined using the Clojure keyword type. A keyword is most commonly used to find values in a collection, especially maps.

You can also use a keyword as a function, instead of the get function.

You can also call a map as if it was a function by using a key as an argument to calling the map as a function.

Uncomment each line in turn to compare to previous examples

(starwars-collection :characters)
;; (:characters starwars-collection)

;; (:ships (starwars-collection :rebels))
;; ((starwars-collection :ships) :rebels)

If you dont get the use of maps and keywords like this, dont worry. Just try to see that this is a short-hand version of the (get map keyword) function call.