Re^2: Preventing unintended list expansion inside hash literals.

Interesting. I certainly agree with

a simple ($key) = %$hash_ref would have done the trick better

for clarity, terseness and, more important, lack of side effects. Unless a hash is huge:

>perl -E "$h{$_} = $_ for 0 .. 5_000_000; say time - $^T; keys %h; say
+ time - $^T; $k = scalar each %h; say time - $^T; say $k"
10
10
10
852569

>perl -E "$h{$_} = $_ for 0 .. 5_000_000; say time - $^T; ($k) = %h; s
+ay time - $^T; say $k"
10
176
4034296
[download]

with equally bad effects on memory. Looks like keys in void context is harmless (performance-wise), but assignment of a hash to single-element list is not

Comment on Re^2: Preventing unintended list expansion inside hash literals. Select or Download Code

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Re^3: Preventing unintended list expansion inside hash literals. by LanX (Saint) on Jan 07, 2017 at 13:38 UTC
I did some more meditation and research... First of all I have to say, that I can't possibly see why one would want to store a "representative key" of a hash in another structure (like the OP did) Unless you want only to check if the hash is populated, then `DB<129> scalar %empty => 0` [download] will do the trick without side effects ( `scalar keys %empty` would reset the iterator). Also the usual trick to copy a hash before iterating (in order to protect the iterator) is far slower than assigning to a one element list (which can't compete with each) `DB<100> $h{$_}=$_ for 1..5e6 DB<101> $t=time; %h2=%h; print time-$t; 124 DB<102> $t=time; ($k)=%h; print time-$t; 19 DB<103> use Time::HiRes qw(time) DB<104> $t=time; ($k)=each %h; print time-$t; 7.70092010498047e-05` [download] BUT I have to say I'm very critical about hashes of that size, because memory consumption can easily freeze your system.š Depending on use case I'd either consider using a database or a hash of hashes (of hashes ...) which is far more "swap friendly". Cheers Rolf _{(addicted to the Perl Programming Language and ☆☆☆☆ :) Je suis Charlie!} š) even emptying a hash with 5e6 entries can take minutes on a netbook.	[reply] [d/l] [select]
Re^3: Preventing unintended list expansion inside hash literals. by LanX (Saint) on Jan 06, 2017 at 20:04 UTC
Good point! Cheers Rolf _{(addicted to the Perl Programming Language and ☆☆☆☆ :) Je suis Charlie!}	[reply]