insertion sort using clojure

Question

I ran into this error when I try to evaluate the last line below: "No message. [Thrown class java.lang.NullPointerException]"

(defn my-insertion-sort [lst]
  (loop [list lst result '()]
    (if-not (seq? list) result
            (recur (rest list) (my-insert (first list) result)))))

(defn my-insert [n lst]
  (cond (nil? lst) (list n)
        (> (first lst) n) (conj lst n)
        :else
        (conj (my-insert n (rest lst)) (first lst))))

(my-insertion-sort '(2 1 3))

Where goes wrong with "my-insertion-sort" function?

unionx · Accepted Answer · 2012-01-26 20:25:51Z

2

(defn my-insertion-sort [lst]
  (loop [list lst result '()]
    (if (empty? list) result
        (recur (rest list) (my-insert (first list) result)))))

(defn my-insert [n lst]
  (cond 
    (empty? lst) (list n)
    (> (first lst) n) (conj lst n)
    :else (conj (my-insert n (rest lst)) (first lst))))

answered Jan 26, 2012 at 20:25

unionx

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3 Comments

lkahtz Over a year ago

okay, this works. So what is the why change (nil? __) and (not (seq? __)) to (empty ?) make this difference?

lkahtz Over a year ago

I summarize this into another question: stackoverflow.com/questions/9025124/…

unionx Over a year ago

You can try (nil? ()) and (empty? ()). You used (rest lst), and (rest ()) is (), not nil.

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