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case

case

type: special form (fsubr)
location: built-in
source file: xlcont.c
Common LISP compatible: similar
supported on: all machines

SYNTAX

(case  <expr>  [ ( <value> <action> ) ... ]  )
        <expr>          -       an expression
        <value>         -       an unevaluated expression or list of unevaluated
                                expressions to compare against <expr>
        <action>        -       an expression 

DESCRIPTION

The CASE special form is a selection control form. CASE evaluates <expr>. This value is then compared against all the <value> entries. If <value> is a single atom, the atom is compared against <expr>. If <value> is a list, each of the elements of the list are compared against <expr>. The <action> associated with the first <value> that matches <expr> is evaluated and returned as CASE's result. If no <value> matches, a NIL is returned. If the last <value> is the T symbol and no other <value> has matched <expr>, then CASE will evaluate the <action> associated with T. If there are multiple T entries, the first is considered to be the end of the CASE.

EXAMPLES

(case 'a ('a "a"))                      ; returns "a"
(case 'a ('b "b"))                      ; returns NIL
(case 9 ( 1 "num") (t "ho") (t "hi"))   ; returns "ho"
(case 'a ((1 2 3 4) "number")           ; 
        ( (a b c d) "alpha"))           ; returns "alpha"
(case 'a)                               ; returns NIL
(case)                                  ; returns NIL
(defun print-what (parm)                ; define a function
  (case (type-of parm)                  ;  check PARM type
        ( flonum   (print "float"))     ;
        ( fixnum   (print "integer"))   ;
        ( string   (print "string"))    ;
        ( cons     (print "list"))      ;
        ( T        (print "other")))    ;  otherwise 
  NIL)                                  ;  and always return NIL
(print-what 1.2)                        ; prints  "float"   returns NIL
(print-what 3)                          ; prints  "integer" returns NIL
(print-what "ab")                       ; prints  "string"  returns NIL
(print-what '(a b))                     ; prints  "list"    returns NIL
(print-what 'a)                         ; prints  "other"   returns NIL

NOTE: The CASE special form does not work with a list or string as the <expr>. This is because CASE defines the test used to be the EQL test which does not work with lists or strings, only symbols and numbers.

COMMON LISP COMPATIBILITY: In XLISP, you can use the value T as the last value to get the effect of 'otherwise'. Common LISP uses the symbol OTHERWISE and T for this. If you are porting in code from Common LISP, be careful to make sure T is used instead of OTHERWISE in CASE statements. (Note that no error will be generated, XLISP will just try to match the <expr> to OTHERWISE.


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