Perl Guru Forums: Perl Programming Help: Frequently Asked Questions: What's the difference between "delete" and "undef"

CGI/Perl Guide | Learning Center | Forums | Advertise | Login

Site Search: in


	MAIN INDEX	SEARCH POSTS	WHO'S ONLINE	LOG IN

Home: Perl Programming Help: Frequently Asked Questions:
What's the difference between "delete" and "undef"

Jasmine
Administrator

Jan 19, 2001, 3:29 PM

Post #1 of 1 (1171 views)

What's the difference between "delete" and "undef"	Can't Post

(From the Perl FAQ)

What's the difference between "delete" and "undef" with hashes?

Hashes are pairs of scalars: the first is the key, the second is the value. The key will be coerced to a string, although the value can be any kind of scalar: string, number, or reference. If a key $key is present in the array, exists($key) will return true. The value for a given key can be undef, in which case $array{$key} will be undef while $exists{$key} will return true. This corresponds to ($key, undef) being in the hash.

Pictures help... here's the %ary table:

Code

          keys  values 
        +------+------+ 
        |  a   |  3   | 
        |  x   |  7   | 
        |  d   |  0   | 
        |  e   |  2   | 
        +------+------+

And these conditions hold

Code

        $ary{'a'}                       is true 
        $ary{'d'}                       is false 
        defined $ary{'d'}               is true 
        defined $ary{'a'}               is true 
        exists $ary{'a'}                is true (perl5 only) 
        grep ($_ eq 'a', keys %ary)     is true

If you now say

Code

        undef $ary{'a'}

your table now reads:

Code

          keys  values 
        +------+------+ 
        |  a   | undef| 
        |  x   |  7   | 
        |  d   |  0   | 
        |  e   |  2   | 
        +------+------+

and these conditions now hold; changes in caps:

Code

        $ary{'a'}                       is FALSE 
        $ary{'d'}                       is false 
        defined $ary{'d'}               is true 
        defined $ary{'a'}               is FALSE 
        exists $ary{'a'}                is true (perl5 only) 
        grep ($_ eq 'a', keys %ary)     is true

Notice the last two: you have an undef value, but a defined key!

Now, consider this:

Code

        delete $ary{'a'}

your table now reads:

Code

          keys  values 
        +------+------+ 
        |  x   |  7   | 
        |  d   |  0   | 
        |  e   |  2   | 
        +------+------+

and these conditions now hold; changes in caps:

Code

        $ary{'a'}                       is false 
        $ary{'d'}                       is false 
        defined $ary{'d'}               is true 
        defined $ary{'a'}               is false 
        exists $ary{'a'}                is FALSE (perl5 only) 
        grep ($_ eq 'a', keys %ary)     is FALSE

See, the whole entry is gone!

Search for (options)

Web Applications & Managed Hosting Powered by Gossamer Threads
Visit our Mailing List Archives