Clojure returning a list

Question

I know this example is trivial because I think there is a clojure library function that will do this, that is not the point.

I have the following code

(defn makelistres [a b res]
  (if (= a b) res (makelistres a (dec b) (conj res b))))

(defn makelist [a b]
  (makelistres a b [])
)

Is there a way to do the same effect without having to pass the list as a parameter? Such as throwing it on the stack

Such as

(defn test [a b]
  (if (= a b) 0 (+ 1 (test a (dec b))))
)

Don't know if the parenthesis match up, as I wrote this in this text box, but you get the point.

It isn't clear to me what you mean, but you can certainly avoid having a res parameter by using a loop instead. if you are instead concerned about recursive calls consuming stack, recur can avoid this. — Timothy Pratley
– Timothy Pratley, Commented Feb 10, 2016 at 20:09
Yes the ultimate goal is to avoid the res parameter, is there a way to do so without loop? — John Hamlett IV
– John Hamlett IV, Commented Feb 10, 2016 at 20:14
What is it your makelistres function does? I'm sure it works, but I'd just like to know what to expect when running it at the REPL. — Chris Murphy
– Chris Murphy, Commented Feb 10, 2016 at 20:33
i guess it s like (vec (range b a -1)) but with stack overflow if a > b — leetwinski
– leetwinski, Commented Feb 10, 2016 at 20:48

leetwinski · Accepted Answer · 2016-02-10 22:16:28Z

4

is it an option to add one more arity to a function, which will then call the function with additional arg (empty vector) ?

like this:

(defn makelist
  ([a b] (makelist a b []))
  ([a b res]
    (if (== a b) res (makelist a (dec b) (conj res b)))))

in repl:

user> (makelist 1 10)
[10 9 8 7 6 5 4 3 2]

if not, the loop will do:

(defn makelist [a b]
  (loop [a a b b res []]
    (if (== a b)
      res
      (recur a (dec b) (conj res b)))))

or like this (since a is not changed):

(defn makelist [a b]
  (loop [b b res []]
    (if (== a b)
      res
      (recur (dec b) (conj res b)))))

but yeah, there is a lib function for that: (range 10 1 -1)

edited Feb 10, 2016 at 22:16

answered Feb 10, 2016 at 20:41

leetwinski

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Comments

Frank C. · Accepted Answer · 2016-02-16 18:04:04Z

-2

John

There are a couple of ways:

(def res [1,2,3,4,5])

However this requires a priori definition of the data before calling makelistres. There are ways to do this.

(def res-val (atom [])

This is more flexible using atom based functions to set and retrieve programmatically. However; you are using a mutable (stateful) object. Your ideology may shun this!

Because you are changing the content of res the atom approach seems more suited. You would basically set the value of the res (using swap) item from makelist and derefing (e.g. @res) it or modifying it in the recursion of makelistres.

edited Feb 16, 2016 at 18:04

answered Feb 10, 2016 at 21:08

Frank C.

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