Bristle Software Perl Tips

Table of Contents:

Details of Tips:

1. Passing parameters to subroutines

   Last Updated: 10/6/1999
   Applies to:  Perl 5

   Perl subroutine calls with parameters look like any of the following:

   	sub1 1, "abc", 2;
   	&sub1 1, "abc", 2;
   	sub1 (1, "abc", 2);
   	&sub1 (1, "abc", 2);

   The ampersand ("&") and the parentheses are optional in most cases.  The called subroutine can then refer to the parameters as elements of the array @_ as:

   	print "Parameter 1 is: $_[0]";
   	print "Parameter 2 is: $_[1]";
   	print "Parameter 3 is: $_[2]";

   Or it can copy the array into a list of local variables as:

   	my ($param1, $param2, $param3) = @_;
   
   	print "Parameter 1 is: $param1";
   	print "Parameter 2 is: $param2";
   	print "Parameter 3 is: $param3";

   Or it can shift the parameters into local variables as:

   	my $param1 = shift;
   	my $param2 = shift;
   	my $param3 = shift;

   	print "Parameter 1 is: $param1";
   	print "Parameter 2 is: $param2";
   	print "Parameter 3 is: $param3";

   To supply default arguments for the parameters, in case the caller omitted some or all of them:

   	my $param1 = ($_ = shift) ? $_ : 123;
   	my $param2 = ($_ = shift) ? $_ : "abcdef";
   	my $param3 = ($_ = shift) ? $_ : 456;

   	print "Parameter 1 is: $param1";
   	print "Parameter 2 is: $param2";
   	print "Parameter 3 is: $param3";

   --Fred

2. Beware ampersand on call without parameters

   Last Updated: 10/10/1999
   Applies to:  Perl 5

   Perl subroutine calls without parameters look like any of the following:

   	sub1;
   	&sub1;
   	sub1();
   	&sub1();

   However, one of these is not like the others.   The 2nd call -- the one with an ampersand ("&") and no parentheses -- has the interesting behavior of passing along the parameter list that was passed to the caller, minus any "shift"ed parameters.  For example:

   	sub a
   	{
   	    my $arg1 = shift;
   	    print "arg1 of a = \"$arg1\"\n";
   	}
   	sub b
   	{
   	    my $arg1 = shift;
   	    print "arg1 of b = \"$arg1\"\n";
   	    &a;
   	}
   	print "Calling b...\n";
   	b 1,2;

   Here b is called with 2 parameters.  It shifts one of them into its $arg1 variable, and then calls a, apparently passing no parameters.   Inside a however, the first parameter is the value passed as the second parameter to b. 

   This is not usually what you want, but it is not a bug.  It is the documented intended behavior of Perl.  To prevent b's parameters from being implicitly passed to a, either remove the ampersand from the call to a or add parentheses to the call.

   --Fred

3. Accessing a database via DBI

   Last Updated: 10/17/1999
   Applies to:  Perl 5

   Database access from Perl is easy using the DBI package.  For example, to read data from an Oracle database:

   use DBI;
   my $drh = DBI->install_driver('Oracle');
   if (! $drh) { print("Error: $DBI::errstr"); }
   my $dbh = $drh->connect("mydatabase", "myuserid", "mypassword");
   if (! $dbh) { print("Error: $DBI::errstr"); }
   my $sth = $dbh->prepare(
   		"select name, SSN, phone_number from person");
   if (! $sth) { print("Error: $DBI::errstr"); }
   my $rv = $sth->execute;
   if (! $rv) { print("Error: $DBI::errstr"); }
   while(($Name, $SSN, $Phone) = $sth->fetchrow)
   {
       DoSomethingWith($Name, $SSN, $Phone);
   }
   my $rv = $sth->finish;
   if (! $rv) { print("Error: $DBI::errstr"); }
   $rv = $dbh->disconnect;
   if (! $rv) { print("Error: $DBI::errstr"); }

   For more info on the DBI package, see:
   
               http://www.symbolstone.org/technology/perl/DBI/index.html

   --Fred