Beefy Boxes and Bandwidth Generously Provided by pair Networks
Just another Perl shrine
 
PerlMonks  

Re: Deleting undef values from a hash

by davido (Archbishop)
on Dec 14, 2003 at 07:47 UTC ( #314619=note: print w/ replies, xml ) Need Help??


in reply to Deleting undef values from a hash

You make an interesting observation, and it sounds like a documentation patch may be in order, as you suggest, liz.

<BEGIN_TANGENT>

I wanted to follow up by further exploring the exists, defined, and each functions.

First, exists:

The POD for exists states that 'exists' returns truth if a hash or array element has been initialized, even if its value is 'undef'.

Example:

my %hash = ('First' => 1, 'Second' => undef ); print "First\n" if exists $hash{'First'}; print "Second\n" if exists $hash{'Second'}; print "Third\n" if exists $hash{'Third'}; __OUTPUT__ First Second

So if the hash element's value is undefined, but the element exists, exists correctly detects the existance of an element.

defined can be a little tricky if you don't think it through. For existant keys with undef value, defined will return false, telling you that 'Second' is not defined. However, defined will also return false for elements that don't exist ('Third', for example). So defined is not the right way to check for the existance of an element, because it will return false for nonexistant elements and for existant elements with undef value. This is old news, but worth mentioning in such discussions as this.

Now for each (the real point to this followup): each knows which elements exist, even if the element's value is undef. So iterating over a hash with each will iterate over all existant elements, regardless of their value (or lack thereof). The POD says it's generally a bad idea to add or delete elements while iterating over a hash with each. But there is an exception though, which is also documented in the POD. Per the POD for each: It is always safe to delete the item most recently returned by each()...

It turns out this is useful. Consider the following code which will remove existing hash elements with undef value:

while ( my ( $key, $val ) = each %hash ) { delete $hash{$key} if not defined $val; }

</END_TANGENT>

Cheers!


Dave


Comment on Re: Deleting undef values from a hash
Select or Download Code

Log In?
Username:
Password:

What's my password?
Create A New User
Node Status?
node history
Node Type: note [id://314619]
help
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others surveying the Monastery: (10)
As of 2014-09-30 13:23 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?

    How do you remember the number of days in each month?











    Results (370 votes), past polls