Create Clojure Map, advanced methods, PART 2

Let’s continue our journey of functions that return maps.

For those missed the first chapters:

In this chapter I am going to talk about, bean, frequencies, group-by and finally clojure.set/index.

Let’s start with


if you don’t take advantage of the clojure compatibility with java, bean is quite useless to you. However if you do import java libraries in your clojure code bean can be one of your best friends.

Given a java object, bean returns a “map” that represents such object.

The best example I could figure out is the one from the clojure doc.

user> (def now (Date.))
user> now
#inst "2014-09-17T06:56:43.345-00:00"
user> (def bean-map (bean now))
user> bean-map
{:day 3, :date 17, :time 1410937003345, :month 8, :seconds 43, :year 114, :class java.util.Date, :timezoneOffset -120, :hours 8, :minutes 56}
user> (.setDate now 25) ;; attention here
user> now
#inst "2014-09-25T06:56:43.345-00:00"
user> bean-map ;; please note the field `date`
{:day 4, :date 25, :time 1411628203345, :month 8, :seconds 43, :year 114, :class java.util.Date, :timezoneOffset -120, :hours 8, :minutes 56}

Please, be very careful here.

What bean returns is not an actual map but something more complex that implements the map protocol.

What you need to know is that changing the underneath original object will do change the “map” that bean returns, so be very careful.


As the name suggests, given a sequence, frequencies returns a map with the frequency of each object in the sequence itself.

user> (frequencies [:a :b :a :a :b :c :d :a :b])
{:a 4, :b 3, :c 1, :d 1} ;; there are 4 :as, 3 :bs, one :c and one :d
user> (frequencies (take 100 (repeatedly #(rand-int 4))))
{3 24, 2 27, 0 15, 1 34} ;; pretty random

If you have understood frequencies


is similar. Let me show you a simple example.

user> (odd? 1)
true ;; 1 is odd
user> (odd? 2)
false ;; 2 is even
user> (range 10)
(0 1 2 3 4 5 6 7 8 9)
user> (group-by odd? (range 10))
{false [0 2 4 6 8], true [1 3 5 7 9]}
user> (defn odd-but-3 [n]
	(if (== 3 n)
		(odd? n)))
user> (group-by odd-but-3 (range 10))
{false [0 2 4 6 8], true [1 5 7 9], "Three" [3]}

As you can see group-by calls the function you provide for every element in the sequence. It stores the output as a key and, as a value, the collection of those elements who returns the same output.


This function is a little complex.

First and foremost we are working with sets, as you may know sets are collections of unique unordered elements.

user> #{:a :b :c}
#{:c :b :a}
user> (clojure.set/union #{:a :b :c} #{:a :h :g})
#{:g :c :h :b :a} ;; no repetition of :a

It is very fast to add element to a set or to check if an element is in the set.

user> (contains? #{:a :b} :a)
user> (contains? #{:a :b} :c)

Of course sets may also contain maps.

user> #{ {:a 1 :b 2} {:a 1 :b 3} {:a 2 :b 2} }
#{{:b 2, :a 1} {:b 3, :a 1} {:b 2, :a 2}}

If I have a big set of maps I may want to explore those maps more deeply and I may need to aggregate together all the maps with the same value for some particular key, this is clojure.set/index.

user> (require '[clojure.set :refer [index]])
nil ;; or call clojure.set/index
user> (def my-set #{ {:a 1 :b 2} {:a 2 :b 1} {:a 1 :b 1} })
user> (index my-set [:a]) ;; aggregate for :a
 {:a 2} #{ {:b 1, :a 2} }, ;; set of one map
 {:a 1} #{ {:b 2, :a 1} {:b 1, :a 1} } ;; set of two maps
user> (index my-set [:b]) ;; aggregate for :b
 {:b 1} #{{:b 1, :a 2} {:b 1, :a 1}},
 {:b 2} #{{:b 2, :a 1}}
user> (index my-set [:a :b])
 {:b 1, :a 1} #{{:b 1, :a 1}},
 {:b 1, :a 2} #{{:b 1, :a 1}},
 {:b 2, :a 1} #{{:b 2, :a 1}}

index is very complex to explain thus I am afraid that I would create more confusion than other trying to word what you can see in action.

If you aren’t completely sure about index you may try to run the function inside a repl: maybe a bigger and more etereogeneous map will help.

As always if you have any question please write a comment bellow.


This chapter was very short and probably somehow boring, but it was necessary for the sake of completeness.

Next time we are moving on to something way more fun and useful.

Next Chapter, Read Clojure Map overview of: get, get-in, contains? and find

As always, stay tuned and write to me if you have anything to say (Ex. Some question, a request for an article, anything…)

I am available for freelance work, I am specialized in IoT and distributed fault tolerant systems, if you are interested in working with me you can get in touch here: simone [at] mweb [dot] biz