在perl中使用constant编译指示允许使用常量。常量顾名思义就是在整个程序的执行过程中,变量的值不会发生变化。如果在编写程序的时候, 你能够确定哪些变量的值不在发生变化,你可以使用use constant的方法来定义常量。下面就是标量常量,列表常量,hash常量的使用方法。
Scalar constant
use constant COUNT = > 100; # ------------------------------------ print COUNT ;List constant
use constant WEEKDAYS = > qw ( Sunday Monday Tuesday Wednesday Thursday Friday Saturday ); # ------------------------------------------------------------------------ print "Today is ",(WEEKDAYS ) [ 1 ],".\n";
Hash constant
use constant WEEKABBR = > { ‘Monday’ = > ‘Mon’,‘Tuesday’ = > ‘Tue’,‘Wednesday’ = >’Wed’,‘Thursday’ = > ‘Thu’,‘Friday’ = >’Fri’ ) # ------------------------------------------------------------------------ %abbr = WEEKABBR; $day = ‘Wednesday’; print “The abbrevaiation for $day is ”,$abbr { $day };