Hm. scalar keys %hash does not seem to be O(N): 

[0] Perl> %a = 1..1e6;;
[0] Perl> %b = ();;
[0] Perl> cmpthese -1, { 
    A=>q[ scalar keys %a and ++$i for 1 .. 1000 ], 
    B=>q[ scalar keys %b and ++$j for 1 .. 1000 ] 
};;
    Rate    A    B
A 8354/s   -- -15%
B 9845/s  18%   --

[0] Perl> %b = (1..2e6);;
[0] Perl> cmpthese -1, { 
    A=>q[ scalar keys %a and ++$i for 1 .. 1000 ], 
    B=>q[ scalar keys %b and ++$j for 1 .. 1000 ] 
};;
   Rate   A   B
A 821/s  -- -0%
B 824/s  0%  --

[0] Perl> %a = ();;
[0] Perl> cmpthese -1, { 
    A=>q[ scalar keys %a and ++$i for 1 .. 1000 ], 
    B=>q[ scalar keys %b and ++$j for 1 .. 1000 ] 
};;
   Rate    B    A
B 815/s   -- -18%
A 998/s  23%   --

[0] Perl> %a = (1,2);;
[0] Perl> cmpthese -1, { 
    A=>q[ scalar keys %a and ++$i for 1 .. 1000 ], 
    B=>q[ scalar keys %b and ++$j for 1 .. 1000 ] 
};;
   Rate   A   B
A 843/s  -- -1%
B 849/s  1%  --

[0] Perl> cmpthese -1, { 
    A=>q[ scalar keys %a and ++$i for 1 .. 1000 ], 
    B=>q[ scalar keys %b and ++$j for 1 .. 1000 ] 
};;
   Rate   B   A
B 804/s  -- -3%
A 831/s  3%  --

[download]

I'll let you draw your own conclusions, but mine are that if the hash is completely empty it is slightly faster than if it has any keys. But whether it has 1 key or 1 million make no difference at all.

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
"Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."

In reply to Re: What is the most efficient way to see if a hash is empty? by BrowserUk
in thread What is the most efficient way to see if a hash is empty? by ELISHEVA

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