Maps (part 2)

The preceding page used strings as keys for the map. This is a typical use of maps in most programming languages. In ClojureScript, maps are also used to create data structures. Instead of using strings as keys, they use keywords. A keyword is a symbol name preceded by a colon, and they are widely used in ClojureScript.

Let’s say you wanted to a data structure to describe a Canadian province that would show the province name, its capital, its population, and its latitude and longitude (as a vector of two numbers). You could set up a map like this for Ontario:

You can use get to access any part of the data:

If you use keywords as your map’s keys, you can also use the keys as if they were a function name. Here is the equivalent of the preceding example:

Modifying maps with assoc

You can use assoc to create a new map with an updated part of the data structure. assoc takes as its arguments the name of the map, the key, and the new value. You can have more than one key and value pair after the map name. Thus, to create a new map with a modified population, you could do this:

It is possible to have maps within maps. Consider this data structure for an event:

You can retrieve the state, for example, by doing either of the following.

If you have a deeply-nested structure, though, this can get messy. This is why ClojureScript provides the get-in function. You give the name of your map followed by a vector of keywords you want to access:

Closely allied with get-in is assoc-in, which lets you easily create a modified nested map. If there were no get-in or assoc-in, you would need to do something clunky like this to modify the email (and you have no idea how long it took me to figure it out):

(assoc event :contact (assoc (:contact event) :email ""))

It is much easier with assoc-in:

If you give a key and value that aren’t in the map, they will be added. So, given the definition of the event, see if you can write an expresson that will add a :price key to the event. The value for that key will be a map with a key :adult and value 7.50. The resulting map will be:

{:date "2017-10-02"
 :name "Annual Talent Show"
 :location {:name "Smith High School"
            :street "300 Main Street"
            :city "Anytown"
            :state "CA"
            :zip "00000"}
 :contact {:name "Joe Doakes"
           :email ""
           :phone "408-555-1234"}
 :price {:adult 7.50}}

The original event has already been defined for you.

Modifying maps with update

The update and update-in functions are similar to assoc and assoc-in, in that they create modified versions of the original map. Instead of key and value pairs, you give a key and a function name and, if necessary, other arguments. The function is applied to the key and the arguments. If you wanted to chnge the province map for a 10% increase in population, you could do this:

You could do this with assoc, but it would be clunkier code:

(assoc province :population (* 1.1 (:population province)))

Similarly, you can use update-in to update a value in a nested map. If, for some unknown reason, you wanted to change the name of the city for the event to all uppercase, you could do this:

So, what is the difference between assoc and update? If you want to replace a value with a new one, use assoc. If you want to perform some operation on a value to create a new value, use update.

Next Section - Atoms