FAQ 4.61 What's the difference between "delete" and "undef" with hashes?
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4.61: What's the difference between "delete" and "undef" with hashes?
Hashes contain 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 $key is
present in %hash, "exists($hash{$key})" will return true. The value for
a given key can be "undef", in which case $hash{$key} will be "undef"
while "exists $hash{$key}" will return true. This corresponds to ($key,
"undef") being in the hash.
Pictures help... here's the %hash table:
keys values
+------+------+
| a | 3 |
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
And these conditions hold
$hash{'a'} is true
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is true
exists $hash{'a'} is true (Perl 5 only)
grep ($_ eq 'a', keys %hash) is true
If you now say
undef $hash{'a'}
your table now reads:
keys values
+------+------+
| a | undef|
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
and these conditions now hold; changes in caps:
$hash{'a'} is FALSE
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is FALSE
exists $hash{'a'} is true (Perl 5 only)
grep ($_ eq 'a', keys %hash) is true
Notice the last two: you have an undef value, but a defined key!
Now, consider this:
delete $hash{'a'}
your table now reads:
keys values
+------+------+
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+
and these conditions now hold; changes in caps:
$hash{'a'} is false
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is false
exists $hash{'a'} is FALSE (Perl 5 only)
grep ($_ eq 'a', keys %hash) is FALSE
See, the whole entry is gone!
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Hans Mulder
> 4.61: What's the difference between "delete" and "undef" with hashes?
[....]
> exists $hash{'a'} is true (Perl 5 only)
I think "Perl 5 only" here means "only if you use Perl 5.000 or later".
Most of the advice given in the FAQ doesn't work in Perl 4, but people
don't complain, because the readers of this newsgroup all switched to
Perl 5 long ago. So I propose to leave out this "Perl 5 only" note
(here and twice more in the same answer).
-- HansM
brian d foy
<han...@xs4all.nl> wrote:
The "Perl 5 Only" isn't hurting anyone or distorting any information,
so I'll leave it in the answer. Beleive it or not, some people are
still using Perl 4. I run into it occassionally.