ClassCastException in Clojure

Question

I am new to clojure and i have been trying out different programs. Here is my program :

(defn sdsu-reverse [x]
  (loop [n (count x) x x]
  (if (= 0 n)
    (x)
    (recur (- n 1) (conj (next x) (first x))))))

(= (sdsu-reverse [1 2 3 4 5]) [5 4 3 2 1])

I have been getting error : java.lang.ClassCastException: clojure.lang.Cons cannot be cast to clojure.lang.IFn C:\Users\Shalima\Documents\Textbooks\Functional Programming\Programs\sample.clj:44 user/sdsu-reverse

But I cant seem to figure it out. Could you please help me with this ?

Thank you.

(x) tries to call x (which is a sequence) as a function. All you need there is x. — Alex
– Alex, Commented Sep 11, 2014 at 18:42
Also (since you're learning) you don't need to count your sequence every time. You just want to check if it's empty. — Diego Basch
– Diego Basch, Commented Sep 11, 2014 at 18:55
@DiegoBasch Indeed you just want to check if it's empty, but note that the original code only counts it once, not every time. — amalloy
– amalloy, Commented Sep 11, 2014 at 19:00
Correct, I thought I saw a count in the comparison. The OP's code (as it is now at least) counts at the start of the loop. It does keep an unnecessary counter though. — Diego Basch
– Diego Basch, Commented Sep 11, 2014 at 19:14

Chris · Accepted Answer · 2014-09-12 21:29:09Z

2

As Alex notes above, you need to replace (x) with x in your if expression. Wrapping x in parentheses treats it as a function when instead you want to return a value.

As for the other issue in your code:

conj is a slightly confusing function.

clojure.core/cons
([x seq])
  Returns a new seq where x is the first element and seq is
    the rest.

Sounds clear enough. But look at what happens when conj is applied to a list versus a vector (source).

user=> (conj [1 2 3] 4)
[1 2 3 4]

user=> (conj '(1 2 3) 4)
(4 1 2 3)

The reason for this has to do with how the vector and list datatypes are constructed. Vectors receive new values from the righthand side; lists, from the lefthand side.

When you call conj (next x) (first x), you effectively are calling conj '(2 3 4) '(1), over and over again. So you end up with the same value that you started at.

A better approach would be to use recursion as follows.

(defn sdsu-reverse [x]
  (if (empty? x)
    nil
    (cons (last x) (sdsu-reverse (drop-last x)))))

I hope this helps.

Edit in response to Leonid's comments.

Leonid is correct that the above will fail for large sequences. Alternatively you could do something like

(defn reverse' 
  ([x] (reverse' x nil))
  ([x acc] (if (empty? x) 
             acc 
             (recur (rest x) (cons (first x) acc)))))

edited Sep 12, 2014 at 21:29

answered Sep 11, 2014 at 23:43

Chris

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4 Comments

Leonid Beschastny Over a year ago

Using recur is always better than using direct recursion, not only because it's much faster, but also because direct recursion leads to stack overflow. Try reversing something like (range 99999) and you'll see what I mean.

Leonid Beschastny Over a year ago

Using last and drop-last instead of first and rest is also a very bad idea, because they are extremely ineffective when applied to sequences.

Chris Over a year ago

Agreed. Amended my answer.

Leonid Beschastny Over a year ago

Just for the sake of completeness: the simplest and the most efficient way to revert a sequence is to use reduce higher-order function: (reduce conj () x), which is exactly how build-in reverse function works.

piyushmandovra · Accepted Answer · 2014-09-12 06:35:08Z

0

class cast exception in your code is because you try to make a list from a constant i.e. (x) so just replace it with x you won't get class cast exception.

answered Sep 12, 2014 at 6:35

piyushmandovra

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