Vectors and Maps are Only Different in Your Mind

"That roars so loud and thunders in the index?"

-- Shakespeare, Hamlet

Act III, Scene IV, Line 53

The other day I found something interesting while reading Clojure Programming.

"While it might be counterintuitive, maps and vectors are both associative collections, where vectors associate values with indices."

-- Clojure Programming, page 100

This is yet another case of the book, Clojure Programming, stating something that I thought of but was unable to adequately express in words.
The differences placed around data structures are often either just in our minds or a condition you place on the structure its self.

In the case of the structures just being different in our minds we can look at the example of a map and vector in Clojure.

	;; vector
	(def v [1 2 3 4])
	;; #'user/v
	(get v 1)
	;; 2
	(get v 99 :not-found)
	;; :not-found
	(v 0)
	;; 1
	(v 99)
	;; IndexOutOfBoundsException clojure.lang.PersistentVector.arrayFor (PersistentVector.java:107)
	(assoc v
	1 "Mr. Jack"
	0 2002
	2 :good-boy)
	;; [2002 "Mr. Jack" :good-boy 4]
	;; map
	(def m {0 1 1 2 2 3 3 4})
	;; #'user/m
	(get m 1)
	;; 2
	(get m 99 :not-found)
	;; :not-found
	(m 0)
	;; 1
	(m 99)
	;; nil
	(assoc m
	1 "Mr. Jack"
	0 2002
	2 :good-boy)
	;; {0 2002, 1 "Mr. Jack", 3 4, 2 :good-boy}

view raw repl.clj hosted with ❤ by GitHub

We see in the example above that we could use a vector as a map where the indices are the keys.  We see in the first example (get v 1) that we are just using the index of 1 to get the second value from the vector (indices are zero based in Clojure).  With the second example (get v 99 :not-found) we see that we can assign a default value to be return when a value is not found.  In the last vector example we see that we can associate new values in the vector resulting in a different vector being returned (do not worry vectors are immutable in Clojure).  In the map examples we see we can treat a map like a vector by using integers for the keys.

In the second case of data structures being different due to a condition imposed on them, we do not have to look an further than our example of a map.

(assoc m


  1 "Mr. Jack"


  0 2002


  2 :good-boy)


;; {0 2002, 1 "Mr. Jack", 3 4, 2 :good-boy}

We see a map is unordered in Clojure.  If you want to have the data be stored in an order then you would want to use a sorted-map instead of a hash-map.