Thanks for the direction, For anyone who's interested, I found an excellent article that explains this behaviour here:
http://tlc.perlarchive.com/articles/perl/ug0002.shtml Brian -----Original Message----- From: Charles K. Clarkson [mailto:[EMAIL PROTECTED] Sent: 27 July 2004 11:06 To: Brian Ling; [EMAIL PROTECTED] Subject: RE: Help with hash refs Brian Ling wrote: > my $test1 = ${refer}->{value}->{fred}; > print Dumper($refer); > # this doesn't change the data as i'd expect > > my $test2 = ${refer}->{value}->{fred}->{value}; > print Dumper($refer); > # this actually creates a key 'fred' pointing > # to a empty hash ref #I > > So my question really has two parts. > > Why does the above actually change the data, any > pointers to reading material would be great. I've read > a bit about autovification, is that what's going on. > If so, how is this happening with an "assignment" or > an "if". > my $test1 = ${refer}->{value}->{fred}; Perl does not autovivify $refer->{value}{fred}. Read perlfaq4: "Why does passing a subroutine an undefined element in a hash create it?" "Normally, merely accessing a key's value for a nonexistent key does not cause that key to be forever there." > my $test2 = ${refer}->{value}->{fred}->{value}; Here we are looking for a key's value. Perl does not autovivify that key, but it does autovivify $refer->{value}{fred} as expected. > Given that I have a data structure similar to the > above (returned from a module), of which I don't > know the exact shape. How do I test for the > existence of a key, down a branch without > changing the data. All I could come up with is: > > if (${refer}->{value}->{fred} ) { > my $test3 = ${refer}->{value}->{fred}->{value} > } > > As I know if fred exists there will be a key called > value, and testing for fred doesn't change my data. > Is there a better way of doing this? Read 'perlfunc' for details about the 'exists' function. I would still test $refer->{value}{fred}{value}. if ( exists $refer->{value}{fred} and exists $refer->{value}{fred}{value} ) { my $test3 = $refer->{value}{fred}{value}; } OR: my $test3 = $refer->{value}{fred}{value} if exists $refer->{value}{fred} and exists $refer->{value}{fred}{value}; HTH, Charles K. Clarkson -- Mobile Homes Specialist 254 968-8328 http://www.bbc.co.uk/ - World Wide Wonderland This e-mail (and any attachments) is confidential and may contain personal views which are not the views of the BBC unless specifically stated. If you have received it in error, please delete it from your system. Do not use, copy or disclose the information in any way nor act in reliance on it and notify the sender immediately. Please note that the BBC monitors e-mails sent or received. Further communication will signify your consent to this. -- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] <http://learn.perl.org/> <http://learn.perl.org/first-response>
