4

I'm currently going through the 4clojure Problem 23

My current solution uses recursion to go through the list and append each element to the end of the result of the same function:

(fn self [x] (if (= x []) 
                   x 
             (conj (self (rest x)) (first x))
))

But when I run it against [1 2 3] it gives me (1 2 3)
What I think it should be doing through recursion is:

(conj (conj (conj (conj (conj [] 5) 4) 3) 2) 1)

which does return

[5 4 3 2 1]

But it is exactly the opposite, so I must be missing something. Also, I don't understand why ones return a vector and the other one returns a list.

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6

When you do (rest v) you're getting a list (not a vector), and then conj is appending to the front each time (not the back):

user=> (defn self [v] (if (empty? v) v (conj (self (rest v)) (first v))))
#'user/self
user=> (self [1 2 3])
(1 2 3)
user=> (defn self [v] (if (empty? v) [] (conj (self (rest v)) (first v))))
#'user/self
user=> (self [1 2 3])
[3 2 1]
user=>
user=> (rest [1])
()
user=> (conj '() 2)
(2)
user=> (conj '(2) 1)
(1 2)
user=>
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5 Comments

although I understand now the reason (efficiency) it strikes me as weird having the same name for a function with a different behaviour. But thank you for the explanation, it'll probably stick from now on :)
@Willyfrog: well the behaviour is simply "adding", without saying where (and (doc conj) mentions it). For example you can also conj to, say, a set: (conj #{:a :c} :b)
@Willyfrog You can use cons to always append to the front.
conj adds to the end of a vector or the beginning of a list.

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