Class Meeting: Perl II

Admin

• in class presentations in two weeks ...

Notes

• revisit examples from last time...
  • Here is a solution
  • Here is another... it uses the implicit $_ variable
  • Here is a third... it uses boolean expressions in place of if statements

• try this for more fun... write a Perl script that will
  • take as input a text file called "foo.txt"
  • produce as output a file called "bar.txt"
  • output will be input with any duplicate lines removed
  • remember the Perl mantra: "There's more than one way to do it."

  • here is one solution
  • and another ...
  • and a third
  • and a "second" third

• Show assoc array examples from Cato...
  • keywords need translation table
  • CGI interface... forms fill-ins

• subprograms and parameter passing
  • pass values in via implicitly defined @_ array
  • use "globals" a lot
  • cannot pass back values via parameters, as they are by-value
  • can pass back values with "return", and can return a list/array for multiple values

  • example: Cato header, footer and their use;
    note that cgi_header and cgi_footer take an optional argument.

  • example:

      sub yn {
        local ($prompt) = @_ ;
        print STDOUT $prompt ;
        local($reply) = scalar ( < STDIN > ) ; # local($reply) = < STDIN > ???
        $reply =~ /^y/i ;
      }
      ...
    
      $walk if &yn("I'd walk a mile for the Camel Book.  Would you?");
    

  • example:

      $bestday = &max($mon, $tue, $wed, $thu, $fri) ;
      ...
    
      sub max {
         local($max) = pop(@_) ;
         foreach $foo (@_) {
            $max = $foo if $max < $foo ;
         }
         $max ;
      }
    

• Other useful examples... canonical tasks
  • more on array context:
    open(log, ">>logfile"); # regrettable...
    
    
  • sorting... write your own comparison function
    

      sort ( SUB, LIST )
      sort ( LIST )
      sort SUB LIST
      sort LIST
    

• Here is a largish Perl script for examples

  This is Daisitish, an axiomatic tester used in COMP204