Use of uninitialized value in string comparison (cmp)

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

Good $localtime fellow monks

I have a large piece of code, and while debugging, the output routine is giving me the above message, lots. I am passing it an array of hasrefs (alerts basicaly), and trying to sort by the msg value in the hashrefs. I have distilled the problem to a one-liner. TBH, I expected the problem to drop out elsewhere in my code, surprised I see it in this:

perl -wle '@r=({k=>"r"},{k=>"s"},{k=>"t"});print $_->{k} for sort { $_
+->{k} cmp $_->{k} } @r'
Use of uninitialized value in string comparison (cmp) at -e line 1.
Use of uninitialized value in string comparison (cmp) at -e line 1.
Use of uninitialized value in string comparison (cmp) at -e line 1.
Use of uninitialized value in string comparison (cmp) at -e line 1.
Use of uninitialized value in string comparison (cmp) at -e line 1.
Use of uninitialized value in string comparison (cmp) at -e line 1.
r
s
t
[download]

I get the same result if I deref it this way to:

perl -wle '@r=({k=>"r"},{k=>"s"},{k=>"t"});print $_->{k} for sort { $$_{k} cmp $$_{k} } @r'

Why does this happen, where is it finding unitialised values? and of course, how can I fix it?

Cheers,
R.

Pereant, qui ante nos nostra dixerunt!

Comment on Use of uninitialized value in string comparison (cmp) Select or Download Code

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Re: Use of uninitialized value in string comparison (cmp) by Discipulus (Canon) on Sep 16, 2015 at 10:49 UTC
Hello, sort needs to operate on $a and $b not on $_ `#sorry win quote perl -wle "@r=({k=>'r'},{k=>'s'},{k=>'t'});print $_->{k} for sort { $a +->{k} cmp $b->{k} } @r;" r s t` [download] L* There are no rules, there are no thumbs.. Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.	[reply] [d/l]
Re^2: Use of uninitialized value in string comparison (cmp) by Random_Walk (Prior) on Sep 16, 2015 at 10:57 UTC
Aaarrgghhh, I knew I had been staring at it too long. I knew that, but it slipped out my memcache somewhere :(. Thanks for the reminder. Cheers, R. Pereant, qui ante nos nostra dixerunt!	[reply]


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