clj exercies.md

Some Exercises And Solutions For Beginners

Assuming:

(def  v [{:a 1} {:a 2} {:a 3}])
  
(map f v)

Exercises:

1. write an f which will add one to the value of each :a.
   

   [{:a 2} {:a 3} {:a 4}]

2. write an f which will:
   • add one to :a
   • add a :b key which has a value of 1
     

    [{:a 2 :b 1} {:a 3 :b 1} {:a 4 :b 1}]`

3. write an f which will
   • add one to :a
   • add a :b key which has the original value of :a
     

   [{:a 2 :b 1} {:a 3 :b 2} {:a 4 :b 3}]

4. write an f which will
   • add one to :a
   • add a :b key which has the original value of :a but only if :a is greater than 3
     

   [{:a 2} {:a 3} {:a 4 :b 3}]

Possible solutions supplied below - scroll down when you have given it a try ...

Hey, don't cheat. I can see you. Try before you look.

Solution for #1

Write an f which will add one to the value of each :a.

[{:a 2} {:a 3} {:a 4}]

(defn f 
  [m]
  (update m :a inc))

*solution for #2 further below *

Solution for #2

Write an f which will:

• add one to :a
• add a :b key which has a value of 1
  

 [{:a 2 :b 1} {:a 3 :b 1} {:a 4 :b 1}]

(defn f 
  [m]
  (let [new-m (update m :a inc)]
    (assoc new-m :b 1))

or

a very functional approach

(defn f 
  [m]
  (assoc (update m :a inc) :b 1))

or

This is more typical clojure code, which uses the threding macro -> (which is effectively the same as the solution above)

(defn f 
  [m]
  (-> m
    (update :a inc)
    (assoc :b 1)))

Stop cheating!!! Solution for #3 further below

Solution for #3

Write an f which will:

• add one to :a
• add a :b key which has the original value of :a
  

[{:a 2 :b 1} {:a 3 :b 2} {:a 4 :b 3}]

(defn f 
  [m]
  (let [original-a (:a m)]
    (-> m
      (update :a inc)
      (assoc :b original-a))))

Solution for #4

Write an f which will:

• add one to :a
• add a :b key which has the original value of :a but only if :a is greater than 3
  

[{:a 2} {:a 3} {:a 4 :b 3}]

(defn f 
  [m]
  (let [original-a (:a m)
        new-m    (update m :a inc)]
    (if (> original-a 2)
        (assoc new-m :b original-a)
        new-m)))