Day 89 - Meeting Clojure data science company
A second meeting with Cervest, a very interesting data science company supported by Clojure tooling.
Also met with All Street Research who are looking to disrupt the financial industry and building systems in Clojure.
Solved #9 and #10 challenges from 4Clojure on the train home.
4Clojure Challengesλ︎
9 Conj with setsλ︎
A very simple challenge using conj to add values to a set.
Design Journal
(ns four-clojure.009-sets-conj)
;; #9 Sets: conj
;;
;; Difficulty: Elementary
;; Topics:
;; When operating on a set, the conj function returns a new set with one or more keys "added".
;; (= #{1 2 3 4} (conj #{1 4 3} __))
;; Deconstruct the problem
;;
;; A simple challenge to show how `conj` works with sets.
;; REPL experiments
;;
;; As discussed in challenge #8, sets do not really care about order
;; it is only the values within the set that is important
;; So if the result we want is `#{1 2 3 4}`
;; then we just need to conjoin the missing number
(conj #{1 4 3} 2)
;; => #{1 4 3 2}
;; Answers summary
;;
;; Most readable answer
2
;; Superfluous answers
;; As conj can take multiple values you could add any of the values already in the set,
;; so long as you are adding 2 also.
;; Extra values will just be stripped out.
;; 1 2 3 4
;; 1 2 3 4 1 2 3 4 1 2 3 4
10 Intro to maps -λ︎
A quick look at creating and extracting values from a map.
Design Journal
(ns four-clojure.010-intro-to-maps)
;; #10 Intro to maps
;;
;; Difficulty: Elementary
;; Topics:
;; Maps store key-value pairs. Both maps and keywords can be used as lookup functions. Commas can be used to make maps more readable, but they are not required.
;; (= __ ((hash-map :a 10, :b 20, :c 30) :b))
;; (= __ (:b {:a 10, :b 20, :c 30}))
;; Deconstruct the problem
;;
;; Looking at ways to create a map in Clojure,
;; also know as a hash-map or a dictionary
;; The result can be seen by simply evaluating
;; the right hand side of the expression in the REPL
;; REPL experiments
;;
;; The `hash-map` function will construct a map from pairs of values
;; A map must contain zero or more pairs of values
;; The first value of pair is the key
;; The second value of the pair is a value
;; The key is used to look up the value in the map.
;; Here we create a map using the `hash-map` funtion
(hash-map :a 10 :b 20 :c 30)
;; => {:c 30, :b 20, :a 10}
;; Now we use that map as a function with a key as its argument,
;; returning the value that the key points too.
((hash-map :a 10 :b 20 :c 30) :b)
;; => 20
;; The above code can also be written specifically as:
({:c 30, :b 20, :a 10} :b)
;; => 20
;; Looking at the second test, we can see that the key can also be used as a function call
;; However, this only works when the key is a `keyword`, eg. it starts with a colon, `:`.
(:b {:a 10 :b 20 :c 30})
;; Using the map or keyword as a function call is a short-cut for using the `get` function.
(get {:a 10 :b 20 :c 30} :b)
;; => 20
;; The `get` function is often considered more readable, especially by those still
;; learning Clojure.
;; Using the short form is acceptable as simple in-line functions (lambda functions),
;; particularly where that function is passed to another function as an argument (eg. map, filter)
;; There is a similar function to `get` called `get-in` which elegantly helps you traverse a nested map.
;; You may have noticed I have not used commas `,` in my maps during the REPL experiments,
;; `,` are treated as white-space by Clojure and rarely used by Clojure developers.
;; Answers summary
;;
;; Most readable answer
20
Thank you.
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