CODEWITHSUNDEEP
exceptionclojure
ewell128
ewell128

Reputation: 13

Concatenate lists in Clojure Without Using concat

I am new to clojure, and am trying to understand how to concatenate lists without using the concat function.

(defn append  [list1, list2]
  (cond
    (nil? list1) list2
    :else (cons (first list1)
            (append (rest list1) (list2)))))

When I run the above code, I get:

ClassCastException clojure.lang.PersistentList cannot be cast 
   to clojure.lang.IFn  user/append

Reading from previous stackoverflow questions, it is because the cond expression is in double parentheses. I don't believe that I do.

Why am I still getting this error?

Upvotes: 1

Views: 444

Answers (2)

Gwang-Jin Kim
Gwang-Jin Kim

Reputation: 9865

(defn append
   [l1 l2 & [acc]]
   (cond (and (empty? l1) (empty? l2)) (reverse acc)
         (empty? l1) (append l1 (rest l2) (cons (first l2) acc))
         :else (append (rest l1) l2 (cons (first l1) acc))))

or:

(defn append
  ([l1 l2]
   (append l1 l2 '()))
  ([l1 l2 acc]
   (cond (and (empty? l1) (empty? l2)) (reverse acc)
         (empty? l1) (append l1 (rest l2) (cons (first l2) acc))
         :else (append (rest l1) l2 (cons (first l1) acc)))))

;; or using vectors and conj
(defn vec-append
  [v1 v2]
  (cond (empty? v2) v1
        :else (vec-append (conj v1 (first v2)) (rest v2))))

(defn append
   [l1 l2]
   (seq (vec-append (vec l1) (vec l2))))

Upvotes: 0

Alan Thompson
Alan Thompson

Reputation: 29958

In Clojure, parentheses mean "function call". Thus the expression:

(list2)

means call a function that the list2 variable points at. This is the source of the error and the message clojure.lang.PersistentList cannot be cast to clojure.lang.IFn

Notes:

  1. When a cond has only 2 branches, it is usually clearer to just a plain if expression.
  2. Clojure code ignores commas, so they are normally never included in source code.
  3. Consider carefully the difference between rest and next:
    (rest []) => ()
    (next []) => nil

You may wish to carefully study the Clojure CheatSheet and other documention listed here.

Upvotes: 3

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