clojure - delete an element from a ref vector

Question

I'm using a vector of maps which defined as a referece.

i want to delete a single map from the vector and i know that in order to delete an element from a vector i should use subvec.

my problem is that i couldn't find a way to implement the subvec over a reference vector. i tried to do it using: (dosync (commute v assoc 0 (vec (concat (subvec @v 0 1) (subvec @v 2 5))))), so that the seq returned from the vec function will be located on index 0 of the vector but it didn't work.

does anyone have an idea how to implement this?

thanks

Using a vector to store something you'll want to delete in a random-access fashion is usually the wrong choice - they can't do it efficiently and as a result the language features for doing this with them are awkward. Consider just using a list/seq instead. — amalloy
– amalloy, Commented Feb 15, 2012 at 19:18

mikera · Accepted Answer · 2012-02-15 15:32:44Z

5

commute (just like alter) needs a function that will be applied to the value of the reference.

So you will want something like:

;; define your ref containing a vector
(def v (ref [1 2 3 4 5 6 7]))

;; define a function to delete from a vector at a specified position
(defn delete-element [vc pos]
  (vec (concat 
         (subvec vc 0 pos) 
         (subvec vc (inc pos)))))

;; delete element at position 1 from the ref v
;; note that communte passes the old value of the reference
;; as the first parameter to delete-element
(dosync 
  (commute v delete-element 1))

@v
=> [1 3 4 5 6 7]

Note the that separating out the code to delete an element from the vector is a generally good idea for several reasons:

This function is potentially re-usable elsewhere
It makes your transaction code shorter and more self-desciptive

edited Feb 15, 2012 at 15:32

answered Feb 15, 2012 at 13:22

mikera

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1 Comment

Rafał Dowgird Over a year ago

(count vc) is redundant as the third argument to (subvec vc)

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