Clojure length of sequence

Question

I could have sworn I had alength working previously, but I don't quite know what I am doing wrong right now:

user=> (alength '(1 2 3))
IllegalArgumentException No matching method found: alength  clojure.lang.Reflector.invokeMatchingMethod (Reflector.java:79)
user=> (alength [1 2 3])
IllegalArgumentException No matching method found: alength  clojure.lang.Reflector.invokeMatchingMethod (Reflector.java:79)
user=> (doc alength)
-------------------------
clojure.core/alength
([array])
  Returns the length of the Java array. Works on arrays of all
  types.
nil

What should I be doing to get the length of a list/array in Clojure?

Óscar López · Accepted Answer · 2019-05-20 19:34:34Z

54

Try using count:

(count '(1 2 3))
=> 3
(count [1 2 3])
=> 3

edited May 20, 2019 at 19:34

answered Dec 26, 2011 at 1:49

Óscar López

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Comments

skuro · Accepted Answer · 2011-12-26 01:59:07Z

38

As the docstring says, alength works on Java™ arrays, such as a String[] or Integer[], which is definitely an incompatible type with Clojure lists or vectors, for which you want to use count:

user=> (def x '(1 2 3))
#'user/x
user=> (def xa (to-array x))
#'user/xa
user=> (class x)
clojure.lang.PersistentList
user=> (class xa)
[Ljava.lang.Object;
user=> (alength xa)
3
user=> (alength x) 
java.lang.IllegalArgumentException: No matching method found: alength (NO_SOURCE_FILE:0)
user=> (count x)
3

[Ljava.lang.Object; is the weird way toString is defined to output for native Object arrays.

edited Dec 26, 2011 at 1:59

answered Dec 26, 2011 at 1:48

skuro

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1 Comment

kotarak Over a year ago

Note, that count does also work for arrays. alength is only needed in speed-critical code where you already know that you have an array and need direct access. count would be slower here, because it's more generic.

katspaugh · Accepted Answer · 2011-12-26 01:47:23Z

14

It should be count.

user=> (count '(1 2 3))
3

answered Dec 26, 2011 at 1:47

katspaugh

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Comments

mirabilos · Accepted Answer · 2014-06-02 00:33:12Z

2

This may be overkill, but you could imitate Common LISP's length function like this:

(def length 
 (fn [lst]
  (loop [i lst cnt 0]
   (cond (empty? i) cnt
     :t (recur (rest i)(inc cnt))))))

edited Jun 2, 2014 at 0:33

mirabilos

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answered Jun 2, 2014 at 0:09

user2250294

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Comments

Abner Terribili · Accepted Answer · 2017-07-18 02:15:23Z

2

You can do that using the recursive way:

(defn length
 [list]
 (if (empty? list) 0
  (+ 1 (length (rest list)))))

Hope it helps!

answered Jul 18, 2017 at 2:15

Abner Terribili

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