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case

3.13 Dispatch: case 🔗

syntax

(case val-expr case-clause ...)

 
case-clause = [(datum ...) then-body ...+]
  | [else then-body ...+]
Evaluates val-expr and uses the result to select a case-clause. The selected clause is the first one with a datum whose quoted form is equal? to the result of val-expr. If no such datum is present, the else case-clause is selected; if no else case-clause is present, either, then the result of the case form is #<void>.

The case form of racket differs from that of R6RS or R5RS by being based on equal? instead of eqv? (in addition to allowing internal definitions).

For the selected case-clause, the results of the last then-body, which is in tail position with respect to the case form, are the results for the whole case form.

A case-clause that starts with else must be the last case-clause.

The case form can dispatch to a matching case-clause in O(log N) time for N datums.

Examples:
> (case (+ 7 5)
   [(1 2 3) 'small]
   [(10 11 12) 'big])

'big

> (case (- 7 5)
   [(1 2 3) 'small]
   [(10 11 12) 'big])

'small

> (case (string-append "do" "g")
   [("cat" "dog" "mouse") "animal"]
   [else "mineral or vegetable"])

"animal"

> (case (list 'y 'x)
   [((a b) (x y)) 'forwards]
   [((b a) (y x)) 'backwards])

'backwards

> (case 'x
   [(x) "ex"]
   [('x) "quoted ex"])

"ex"

> (case (list 'quote 'x)
   [(x) "ex"]
   [('x) "quoted ex"])

"quoted ex"

 

(define (classify c)
  (case (char-general-category c)
   [(ll lu lt ln lo) "letter"]
   [(nd nl no) "number"]
   [else "other"]))

 

> (classify #\A)

"letter"

> (classify #\1)

"number"

> (classify #\!)

"other"