Java's switch equivalent in Clojure?

Question

Is there an equivalent for Java's switch construct in Clojure? If yes, what is it? If no, do we have to use if else ladder to achieve it?

score 42 · Answer 1 · edited Jan 08 '14 at 10:37

42

case is a good option as pointed out by Jan

cond is also very useful in many related circumstances, particularly if you want to switch on the basis of evaluating a range of different conditional expressions, e.g.

(defn account-message [balance]
  (cond 
    (< balance 0) "Overdrawn!"
    (< balance 100) "Low balance"
    (> balance 1000000) "Rich as creosote"
    :else "Good balance"))

Note that the result of cond is determined by the first matching expression, so a negative balance will display "Overdrawn!" even though it also matches the low balance case.

[I have edited the code - removed the extra bracket at the end to make it work]

edited Jan 08 '14 at 10:37

monojohnny

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answered Dec 02 '11 at 10:05

mikera

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Nice reference to the Discworld :) – omiel Feb 10 '14 at 04:33
Note that `cond` will return `nil` if none of the conditions are satisfied! – Purplejacket Feb 20 '15 at 23:32

Jan · Accepted Answer · 2011-12-02T06:20:01.567

41

Try the case macro:

(case (+ 2 3)
  6 "error"
  5 "ok")

or with default value

(case (+ 2 3)
  5 "ok"
  "error")

Remember that according to the documentation

The test-constants are not evaluated. They must be compile-time literals, and need not be quoted. (...)

See more examples at ClojureDocs.

edited Dec 02 '11 at 06:20

answered Dec 02 '11 at 06:12

Jan

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If your test values are non-literals, you might also want to look into `condp`: `(condp = (+ 2 3) (inc 4) "Ok" (dec 7) "error")`. – kotarak Dec 02 '11 at 07:00
@Jan Using `case` of clojure is slowing down the process (noticeable time), is there any other alternative? – vikbehal Dec 02 '11 at 11:34
@vikbehal, there's also `cond`, as pointed out in mikera's answer. I'm really interested to learn what might be the cause of this slowdown. Could you do some research, e.g. profiling, and share the results? – Jan Dec 02 '11 at 14:20

omiel · Answer 3 · 2013-12-02T14:05:08.840

Though @Jan and @mikera suggestions to use case or cond (may I add condp to the list?) are sound from a functional¹ standpoint and though case 's limitations (e.g. test values can only be compile-time constants ; a default return value is mandatory) mirror those of switch there are some subtle differences:

case cannot be used with Java Enum constants ;
case 's dispatch is based on hashing AFAIK which makes it comparable to hashmaps in terms of performance ; switch is way faster ;
you cannot fall-through with case, which means that you must use other options (condp with value sets ?) to mirror switch 's behaviour.

[¹] not functional as in functional-programming, functional as in fulfilling a function, serving a purpose.

Java's switch equivalent in Clojure?

3 Answers3