Clojure设定字串值

Question

I have a variable "testtext". Depending on a other variable "testvalue", thats always the value 1 or 2, it needs to be set to something. So if testvalue is 1, i need to set testtext to "its one". And when testvalue is 2, i need to set testtext to "its two". Right now i have:

 (cond
        (= testvalue 1) (var-set testtext "its one")
        (= testvalue 2) (var-set testtext "its two")
        :else (var-set testtext "ERROR")
        )

But i get the error "String cannot be cast to clojure.lang.Var" So my question is, how do i properly set a string value, assuming that's what I did wrong.

Answer 1

You want something more like this:

 (let [result (cond
                (= testvalue 1) "its one"
                (= testvalue 2) "its two"
                :else "ERROR"  ) ]
   (println "result:" result))

Using var-set is very rare in Clojure. I can't answer in too much more detail without knowing your exact use-case.

If you really need something like a Java variable, you could use a Clojure atom :

 (def result (atom nil))
 (cond
   (= testvalue 1) (reset! result "its one")
   (= testvalue 2) (reset! result "its two")
   :else (reset! result "ERROR" ))
 (println "result:" @result))

Answer 2

You probably want something like:

(defn func 
  [test-value]
  (let [test-var (cond
                 (= test-value 1) "it's one"
                 (= test-value 2) "it's two"
                 :else "ERROR")]
    test-var))
=> #'user/func
(func 1)
=> "it's one"
(func 2)
=> "it's two"
(func 3)
=> "ERROR"

The let form let's you assign values to vars. Sticking it in a function is a convenient way to return the result.

Answer 3

On most contexts you don't want to modify an existing "variable's" value, instead you use let to bind a value to a symbol. It would be easier to answer if we knew the bigger picture, but as already mentioned atom is a datatype to which you can atomically swap in a new value.

I'd say this is more idiomatic, although not quite what you asked:

(def lookup-table
  {1 "its one"
   2 "its two"})

(defn make-lookup [i]
  (get lookup-table i "ERROR"))

(doseq [i (range 4)]
  (println i "=" (make-lookup i)))

Prints:

0 = ERROR
1 = its one
2 = its two
3 = ERROR

Instead of thinking how to assign a value to a variable, you should think what do you want to use that value for.