How to change few lists inside single hashmap?

Question

I wonder what is the idiomatic way of doing complicated data structures changes. Here is a hashmap that contains lists, i want to take one of those list and move some items to the other one:

input:

{ :a (1 2 3) :b (4 5) }

output:

{ :a (2 3) :b (4 5 1) }

"first element of :a is added as last to :b"

In practice I need such structure to represent game state like:

{ :first_player { :deck (2 3 4 5) :hand (6 1) :discard () }
  :second_player { :deck (1 8 9 10) :hand (3) :discard (1 7) }
  :board { :first_player_side (1 3) :second_player_side (7 9) }}

As u can see i will need to move cards ids from different lists to different lists in various hashmaps (play from hand to board, move from board to discard pile etc). I just wonder how to make it in simple/readable way.

Thanks for answers.

Please at least provide some code that you have tried on your own before asking others to write code for you — johnbakers, Apr 21 '14 at 10:18
i dont think that is required, there is plenty of questions without example code. — chaos-theory, Apr 21 '14 at 17:25

Leonid Beschastny · Accepted Answer · 2014-04-21T13:50:18.670

3

Clojure has an update-in function, designed for doing complicated data structures changes:

(defn magic [{:keys [a] :as m}]
  (-> m
      (update-in [:a] rest)
      (update-in [:b] concat (take 1 a))))

Or more general solution:

(defn move-els [m from to numel]
  (let [els (take numel (get m from))]
    (-> m
        (update-in [from] (partial drop numel))
        (update-in [to] concat els))))

Try it:

=> (move-els { :a '(1 2 3) :c "foo" :b '(4 5) } :a :b 2)
{:a (3), :c "foo", :b (4 5 1 2)}

edited Apr 21 '14 at 13:50

answered Apr 21 '14 at 13:43

Leonid Beschastny

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clever use of destructuring and update-in indeed does a job well. thanks for your answer. – chaos-theory Apr 22 '14 at 09:41

score 0 · Answer 2 · answered Apr 21 '14 at 11:10

0

Are you looking for map destructuring?

(defn frob [{:keys [a b]}]
    {:a (rest a) :b (conj b (first a))})

This doesn't do exactly what you want, of course, because that wouldn't be much fun.

See http://blog.jayfields.com/2010/07/clojure-destructuring.html for a good quick overview of restructuring.

answered Apr 21 '14 at 11:10

pete23

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Using conj can lead to suprising results if people expect lists and vectors to behave the same. – noisesmith Apr 21 '14 at 19:34
Yes:-) Lovely, isn't it? http://stackoverflow.com/questions/7437833/inconsistency-with-clojures-sequences – pete23 Apr 21 '14 at 19:40

How to change few lists inside single hashmap?

2 Answers2