megan-zk
analysis

Clojure Data Structures

In most cases, the values defined as constants are in the form of one of clojure’s many embedded data structures.

Numbers

Strings

Strings represent text directly.

Strings can only be delineated with double-quotes (not single quotes). In order for a quotation mark to be part of a string, the quote must be escaped with a preceding backslash \.

The str function concatenates multiple values into a string. Example:

(def name "Chewbacca") (str "\"Uggllglglglglglglglll\" - " name) The above returns “Uggllglglglglglglglll” - Chewbacca

Maps

Maps are used to associate or group key-value pairs. They are encased in curley brackets - {}. Map literals take this form:

{ :keyword-one “Value-1” :keyword-two “Value-2” }

Keys can also be associated with an operator or function: {"string-key" +}

Maps can be nested (a map acts as a value associate with a keyword for a parent map):

{:name {:first "John" :last "Smith"}}

Map values can be values of any data type.

An alternative to map literals is using the hash-map function:

(hash-map :a 1 :b 2) This will return {:a 1 :b 2}

Retrieving values within maps

The get function will retrieve a value by its key. The function takes the following order: (get map key)

(get {:a 0 :b 1} :b) This will retrieve the value to the keyword :b, or 1.

An optional argument can be added to the end of the get expression that will tell the program what to return if it cannot find a value for the key given. If this argument is not given, then the program will return nil if there is no value for the key given.

(get {:a 0 :b 1} :c "unicorns?") This returns “unicorns?” as there is no value for the key :c

The get-in function retrieves values within nested maps. They take the following order: (get-in map-with-nested-map [:a :b]) In the above, :b is a key within the map value of the key :a.

(get-in {:a 0 :b {:c 3}} [:b :c]) Returns 3

A map can be treated as a function in the clojure expression, in which a specific key for that map can be an operand to retrieve that value of that key.

({:a 1} :a) Returns 1

Keywords

Keywords are used to define values, primarily in maps. A colon (:) precedes the key. Examples:

:a :name :32

Keywords can be used as a function to look up a value. If no value is found, it will return “nil” unless an optional default value is given as the last operand in the expression.

(:a {:a 1 :b 2 :c 3}) Returns 1

(:c {:a 1 :b 2}) Returns nil

(:c {:a 1 :b 2} "hello world") Returns “hello world”

Vectors

Vectors are zero-indexed arrays.

The get function can be used with a vector to get a value by its index. The order for such an expression is: (get vector index)

(get [1 2 3] 0) Returns 1

Vector values can be of any type.

Vectors can be created with a vector function:

(vector 1 2 3) Returns [1 2 3]

The conj function adds elements to the end of an existing vector:

(conj [1 2 3] 4) Returns [1 2 3 4]

Lists

Like vectors, lists are a linear collection of values, with key differences - for example, the get function doesn’t work with lists (instead use the nth function which is described below).

List literals are forms by inserting all the elements in parentheses, each element separated by a space, and placing a single-quote mark before the first parenthesis:

‘(1 2 3 4)

When evaluted, the program with drop the preceding single-quote.

To retrieve a specified element in the list, use

(nth '(:a :b :c) 0) will return :a

Lists can also be created via the list function.

(list 1 2 3 4) Returns (1 2 3 4)

The conj function can be used with lists, like with maps, but elements are added to the beginning of the list and not the end:

(conj '(1 2 3) 4) Returns (4 1 2 3)

Sets

Sets are collections of unique (in other words, unrepeated) values. They come in two forms: hash sets and sorted sets.

Literal notation of a hash set:

#{“kurt vonnegut” 20 :icicle}

The function hash-set can be used to create a set:

(hash-set 1 1 2 2) Returns #{1 2}

You cannot add a value to a set if the value already exists in the set – the set will not change.

(conj #{:a :b} :b) This will just return #{:a :b}

Create a set from existing vectors and lists using the set function:

(set [3 3 3 4 4]) Returns #{3 4}

The contains? function checks if a value is included within a set. It returns true or false.

(contains? #{:a :b} :a) Returns true

(contains? #{:a :b} 3) Returns false

Using a keyword as the function with a set as its operand will return the keyword if it exists within the set, or nil if it does not:

(:a #{:a :b}) Returns :a

The get function on a set works the same way, except the value you are searching for is the last operand.

see also

references

  1. Higginbotham, Daniel. Clojure for the Brave and True Learn the Ultimate Language and Become a Better Programmer. Daniel Higginbotham, 2015, [https://www.braveclojure.com/clojure-for-the-brave-and-true/].
tags: programming / web_development / clojure /