Re: Accidentally creating hash elements with grep { defined } on a hash slice

Why does testing for definition of a value create a key?

Bad question. It's not defined that creates it.

>perl -MData::Dumper -e"grep { 0 } @h{qw(a b)}; print Dumper \%h;"
$VAR1 = {
          'a' => undef,
          'b' => undef
        };
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grep aliases $_ to the element being tested. That means is needs something to alias to. That causes the element to be created.

Is there a way around the key creation?

Yes, pass the keys to grep, not the values.

my @defined_values = map $benchmarks{$_},
                     grep defined($benchmarks{$_}),
                     @BASELINES;
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or maybe you really want

my @defined_values = map $benchmarks{$_},
                     grep exists($benchmarks{$_}),
                     @BASELINES;
[download]

Comment on Re: Accidentally creating hash elements with grep { defined } on a hash slice Select or Download Code

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Re^2: Accidentally creating hash elements with grep { defined } on a hash slice by ccn (Vicar) on Nov 04, 2008 at 11:35 UTC
And what about `$a, $b` in `sort`? They are aliases also, but keys are not created.	[reply] [d/l] [select]
Re^3: Accidentally creating hash elements with grep { defined } on a hash slice by ikegami (Patriarch) on Nov 04, 2008 at 11:39 UTC
I was just asking myself exactly that question. Turns out `grep` requires lvalues, but `sort` doesn't. `>perl -MO=Concise -e"@b = grep f, @h{@a}" ... a <@> hslice lKM ->b ... >perl -MO=Concise -e"@b = sort @h{@a}" ... a <@> hslice lK ->b ...` [download] Notice the "M" for "Modifiable" in the former. So that begs the question: Why does `grep` require lvalues? The difference is: `>perl -e"@a = grep { $_='!'; 0 } @h{qw(i j)};" >perl -e"@a = sort { $a='!'; 0 } @h{qw(i j)};" Modification of a read-only value attempted at -e line 1.` [download] But as far as I can tell, there is no good reason.	[reply] [d/l] [select]
Re^4: Accidentally creating hash elements with grep { defined } on a hash slice by wol (Hermit) on Nov 04, 2008 at 13:51 UTC
Why does grep require lvalues? So you can suprise people with what this does: `perl -e "@in = (1..5); @out = grep { $_++ } @in; print qq{@in}"` [download] But as far as I can tell, there is no good reason. Oh - a good reason? Sorry, haven't got one of them. Move along please, nothing to see here. -- .sig : File not found.	[reply] [d/l]
Re^5: Accidentally creating hash elements with grep { defined } on a hash slice by ikegami (Patriarch) on Nov 04, 2008 at 15:21 UTC
Re^4: Accidentally creating hash elements with grep { defined } on a hash slice by ferment (Novice) on Nov 04, 2008 at 14:28 UTC
Thanks both for your replies. I understand the problem now :)	[reply]