CODEWITHSUNDEEP
clojure
Jelle
Jelle

Reputation: 798

Clojure edit value of vector within function

I have the variable cards with empty values:

(def cards [0 0 0])

Im trying to set a value of cards (depending on input) to a 3 within a function. Thats why i have the following code: 

(defn parse-int [s]
  (Integer. (re-find  #"\d+" s )))

(defn changevalue [] (assoc cards (parse-int (read-line)) 3)) 
(changevalue)

But when i execute the problem and typ in my input 1, the function doesn't change the value of key 1, and when i print it, its still 0 0 0 . How do I fix this?

Im sure the parse-int works, i got it from: In Clojure how can I convert a String to a number?

Upvotes: 0

Views: 283

Answers (2)

Carcigenicate
Carcigenicate

Reputation: 45745

Firstly, just use Integer/parseInt to parse the input:

(defn change-value [cards]
  (let [s (read-line)]
    (assoc cards (Integer/parseInt s) 3)))

This function returns a copy of cards, with one of its elements changed. Remember, cards is a vector, which is immutable. You can't change it once it's been created.

You wrote "the function doesn't change the value of key 1, and when i print it, its still 0 0 0 . How do I fix this?" You don't fix it, you embrace immutability. Don't try to change cards. Use the modified copy that change-value returns:

(def cards [0 0 0])
(def changed-cards (change-value cards))

(println cards)
(println changed-cards)

If you really, really, really need mutability, you can use an atom, but that shouldn't be your first option. Clojure is a functional language with heavy emphasis on immutability. Go with the flow instead of trying to fight the language.

Upvotes: 4

naoki fujita
naoki fujita

Reputation: 719

Clojure's data structure is almost immutable. In clojure, substitution for mutating values is restricted strictly. If you want mutable data structure, you can use Java's mutable data structure or clojure's {atom,ref,var}.

If you truly want to mutate cards, try do this:

(def cards (atom [0 0 0]))
(defn change-cards! [idx] (swap! cards assoc idx 3))
(change-cards! 2)
@cards
#_"here is no parseint to simplify your problem"

Upvotes: 1

Related Questions