typesetting/quad/qtest/mds/case.md

Simple Dispatch: case

The case form dispatches to a clause by matching the result of an expression to the values for the clause:

(case expr                
  [(datum ...+) body ...+]
  ...)                    

Each datum will be compared to the result of expr using equal?, and then the corresponding bodys are evaluated. The case form can dispatch to the correct clause in O_log N___ time for N datums.

Multiple datums can be supplied for each clause, and the corresponding bodys are evaluated if any of the datums match.

Example:

> (let ([v (random 6)])
    (printf "~a\n" v)  
    (case v            
      [(0) 'zero]      
      [(1) 'one]       
      [(2) 'two]       
      [(3 4 5) 'many]))
0                      
'zero                  

The last clause of a case form can use else, just like cond:

Example:

> (case (random 6)
    [(0) 'zero]   
    [(1) 'one]    
    [(2) 'two]    
    [else 'many]) 
'many             

For more general pattern matching (but without the dispatch-time guarantee), use match, which is introduced in [missing].