Function not returning (Clojure)

Question

I am new to clojure, my function elligble voters is not returning the vector, where am i going wrong.

(def human-db
  [
   {:name "Kanishk" :age 28 :sex "male"}
   {:name "Kanishk1" :age 29 :sex "male"}
   {:name "Kanishk2" :age 0 :sex "female"}
   {:name "Kanishk3" :age 1 :sex "male"}
   {:name "Kanishk4" :age 3 :sex "female"}
   {:name "Kanishk5" :age 3 :sex "male"}
   {:name "Kanishk6" :age 3 :sex "female"}
   {:name "Kanishk7" :age 3 :sex "male"}
   {:name "Kanishk8" :age 3 :sex "female"}])


(defn elligble-voters
  [human-db]
  (reduce (fn
            [new-map map]
            (if (> (:age map) 18)
              (conj new-map map)))  [] human-db))
(elligble-voters human-db)

you have no second branch of if: if age is less than or equals 18 it just returns nil . you need to return the accumulator (new-map) instead: (if ... (conj ...) new-map) — leetwinski
– leetwinski, Commented Aug 10, 2018 at 12:11

Marc O'Morain · Accepted Answer · 2018-08-10 12:15:54Z

2

The problem is with your if expression - there is no else clause, so it is returning nil when the voter is aged less than 18.

The last item in human-db is aged 3, so the if returns nil, and thus reduce returns nil.

This works:

(defn elligble-voters [human-db]
  (reduce (fn [new-map map]
            (if (> (:age map) 18)
              (conj new-map map)
              new-map)) ;; <-- Added `new-map` here
          []
          human-db))

A neater way to express the same function would be like this:

(filter (fn [{:keys [age]}] (> age 18)) human-db)

edited Aug 10, 2018 at 12:15

answered Aug 10, 2018 at 12:10

Marc O'Morain

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Comments

birdspider · Accepted Answer · 2018-08-10 12:17:36Z

0

while reduce if capable of doing what you want, use something more suited for filtering a collection.

i.e. filter

clojure.core/filter ([pred] [pred coll])

Returns a lazy sequence of the items in coll for which (pred item) returns logical true. pred must be free of side-effects. Returns a transducer when no collection is provided.

(defn elligble-voters [human-db]
  (filter #(> (:age %) 18) human-db))

#(> (:age %) 18) is an shorthand for an anonymous function, equivalent to

(fn [x] 
  (> (:age x) 18))

answered Aug 10, 2018 at 12:17

birdspider

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