Exercise 09 - Higher Order Functions

A higher-order function is:

• a function that takes one or more functions as arguments OR
• a function that returns a function

Map

The map function relates a sequence to another

(map (fn [x] (* 4 x)) [1 2 3])
  ;;=> [4 8 12]

(map (fn [x] (* x x)) [1 2 3 4 5])
  ;;=> [1 4 9 16 25]

You can map a named fucntion to each item in a sequence as well

(map nil? [:a :b nil :c :d])
  ;;=> [false false true false false]

Filter

(filter (fn [x] false) '(:anything :goes :here))
  ;;=> ()

(filter (fn [x] true) '(:anything :goes :here))
  ;;=> (:anything :goes :here)

(filter (fn [x] (<= 30)) [10 20 30 40 50 60 70 80])
  ;;=> [10 20 30]

Combine Map and Filter

(map (fn [x] (* x 10)) (filter (fn [x] (<= x 3)) [1 2 3 4 5 6 7 8]))
  ;;=> [10 20 30]

Reduce

Basic syntax for reduce:

• (reduce f val coll)
  • f = function
  • val = Starting Value (Optional)
  • coll = Collection

(reduce (fn [a b] (* a b)) [1 2 3 4])
  ;;=> 24

In the example above, there is no starting value for reduce. Reduce will perform the following:

a	b	(* a b)	Result
1	2	(* 1 2)	2
2	3	(* 2 3)	6
6	4	(* 6 4)	24

(reduce (fn [a b] (* a b)) 100 [1 2 3 4])
  ;;=> 2400

In this example, we do have a starting value of 100. Essentially, it will perform the same exact operation as above, but using 100 as a starting value.

a	b	(* a b)	Result
100	1	(* 100 1)	100
100	2	(* 100 2)	200
200	3	(* 200 3)	600
600	4	(* 600 4)	2400