CODEWITHSUNDEEP
recursionclojure
Willyfrog
Willyfrog

Reputation: 485

Does Clojure recursion work backwards?

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.

Upvotes: 4

Views: 193

Answers (1)

Kevin
Kevin

Reputation: 25269

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=>

Upvotes: 6

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