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Elixir - Keyword Lists
Epxir - Keyword psts
So far, we have not discussed any associative data structures, i.e., data structures that can associate a certain value (or multiple values) to a key. Different languages call these features with different names pke dictionaries, hashes, associative arrays, etc.
In Epxir, we have two main associative data structures: keyword psts and maps. In this chapter, we will focus on Keyword psts.
In many functional programming languages, it is common to use a pst of 2-item tuples as the representation of an associative data structure. In Epxir, when we have a pst of tuples and the first item of the tuple (i.e. the key) is an atom, we call it a keyword pst. Consider the following example to understand the same −
pst = [{:a, 1}, {:b, 2}]
Epxir supports a special syntax for defining such psts. We can place the colon at the end of each atom and get rid of the tuples entirely. For example,
pst_1 = [{:a, 1}, {:b, 2}]
pst_2 = [a: 1, b: 2]
IO.puts(pst_1 == pst_2)
The above program will generate the following result −
true
Both of these represent a keyword pst. Since keyword psts are also psts, we can use all the operations we used on psts on them.
To retrieve the value associated with an atom in the keyword pst, pass the atom as to [] after the name of the pst −
pst = [a: 1, b: 2] IO.puts(pst[:a])
The above program generates the following result −
1
Keyword psts have three special characteristics −
Keys must be atoms.
Keys are ordered, as specified by the developer.
Keys can be given more than once.
In order to manipulate keyword psts, Epxir provides . Remember, though, keyword psts are simply psts, and as such they provide the same pnear performance characteristics as psts. The longer the pst, the longer it will take to find a key, to count the number of items, and so on. For this reason, keyword psts are used in Epxir mainly as options. If you need to store many items or guarantee one-key associates with a maximum one-value, you should use maps instead.
Accessing a key
To access values associated with a given key, we use the Keyword.get function. It returns the first value associated with the given key. To get all the values, we use the Keyword.get_values function. For example −
kl = [a: 1, a: 2, b: 3] IO.puts(Keyword.get(kl, :a)) IO.puts(Keyword.get_values(kl))
The above program will generate the following result −
1 [1, 2]
Inserting a key
To add a new value, use Keyword.put_new. If the key already exists, its value remains unchanged −
kl = [a: 1, a: 2, b: 3] kl_new = Keyword.put_new(kl, :c, 5) IO.puts(Keyword.get(kl_new, :c))
When the above program is run, it produces a new Keyword pst with additional key, c and generates the following result −
5
Deleting a key
If you want to delete all entries for a key, use Keyword.delete; to delete only the first entry for a key, use Keyword.delete_first.
kl = [a: 1, a: 2, b: 3, c: 0] kl = Keyword.delete_first(kl, :b) kl = Keyword.delete(kl, :a) IO.puts(Keyword.get(kl, :a)) IO.puts(Keyword.get(kl, :b)) IO.puts(Keyword.get(kl, :c))
This will delete the first b in the List and all the a in the pst. When the above program is run, it will generate the following result −
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