How to have a subroutine return an undefined hash

loris has asked for the wisdom of the Perl Monks concerning the following question:

Hello Hash Honchos,

I have a subroutine which returns a hash, except when things go wrong, in which case it returns undef. I then want to test the result to see what happened:

my %hash = getHash();

if (%hash == undef) {
  print "undef\n";
} else {
  print "not undef\n";
}
[download]

Those of you in the know will see that this does not do what I expect. Can anyone enlighten me as to what is going on?

Thanks

loris

Comment on How to have a subroutine return an undefined hash Select or Download Code

Replies are listed 'Best First'.
Re: How to have a subroutine return an undefined hash by Fletch (Bishop) on Apr 26, 2005 at 12:50 UTC
Putting aside the question of what an "undefined hash" might be, what you might do is return `undef` on failure and a reference to a hash if all goes well. Aside from being easier to deal with, this will be more efficient since the hash contents won't get flattened into a `LIST` and copied back into the destination hash. `sub getHash { ... if( $hunky_dory ) { return \%ret_value; } else { return undef } } my $hash_ref = getHash( ); unless( defined $hash_ref ) { die "Woe is me\n"; } # ... get on with it ...` [download]	[reply] [d/l] [select]
Re^2: How to have a subroutine return an undefined hash by loris (Hermit) on Apr 27, 2005 at 07:47 UTC
Thanks for the help, Fletch and everyone else. The idea of returning a reference to the hash was the bit of wisdom I was lacking. loris	[reply]
Re: How to have a subroutine return an undefined hash by dragonchild (Archbishop) on Apr 26, 2005 at 12:55 UTC
If you have a failure case, don't `return undef;` - just `return;`. That will DWIM for both scalar and list contexts. (If you don't know contexts, don't worry - it will behave correctly.) Then, if you want to check to see if it's got anything, just `if (%hash) { ... }`. The Perfect is the Enemy of the Good.	[reply] [d/l] [select]
Re: How to have a subroutine return an undefined hash by Roy Johnson (Monsignor) on Apr 26, 2005 at 12:38 UTC
Using numeric equality is not how you check defined-ness. ~~Try `defined`~~. See Fletch's note below. Also note that `getHash` should return `()` rather than `undef` to give you an undefined hash. Returning `undef` will give you a malformed-but-defined hash. Caution: Contents may have been coded under pressure.	[reply] [d/l] [select]
Re: How to have a subroutine return an undefined hash by jbrugger (Parson) on Apr 26, 2005 at 12:43 UTC
Why do you want an undefined hash, why not create an empty one, and return the empty hash (or even better return the reference to it?) "We all agree on the necessity of compromise. We just can't agree on when it's necessary to compromise." - Larry Wall.	[reply]
Re: How to have a subroutine return an undefined hash by samizdat (Vicar) on Apr 26, 2005 at 12:49 UTC
`my %hash = getHash(); if (! defined(%hash)) { print "undef\n"; } else { print "not undef\n"; }` [download]	[reply] [d/l]
Re^2: How to have a subroutine return an undefined hash by Fletch (Bishop) on Apr 26, 2005 at 12:53 UTC
Quoth `perldoc -f defined`: `Use of "defined" on aggregates (hashes and arrays) is depre- cated. It used to report whether memory for that aggregate has ever been allocated. This behavior may disappear in future versions of Perl. You should instead use a simple test for size: if (@an_array) { print "has array elements\n" } if (%a_hash) { print "has hash members\n" }` [download]	[reply] [d/l] [select]
Re^3: How to have a subroutine return an undefined hash by samizdat (Vicar) on Apr 26, 2005 at 12:59 UTC
Gawd! Moving targets, yet. Thank you, Fletch, I stand corrected!	[reply]