in reply to Re^5: What is the most efficient way to see if a hash is empty?
in thread What is the most efficient way to see if a hash is empty?

I got that because the more elements the bigger the difference in performance of empty and full hashes. If all we were doing was checking a member of a structure, why would there be any difference at all related to size? Wouldn't the difference between empty and full be simply the difference between one and two pointer de-references - one to get the allocated bucket count and one to get the used bucket count? I should think that would be at most 200%, not 400% and up. And that's assuming that the code involved in dying consumed only a fraction of the time consumed by fetching one or two variables.

Best, beth

  • Comment on Re^6: What is the most efficient way to see if a hash is empty?

Replies are listed 'Best First'.
Re^7: What is the most efficient way to see if a hash is empty?
by BrowserUk (Patriarch) on Apr 28, 2009 at 12:23 UTC

    See the diagrams for HVs at the bottom of PerlGuts Illustrated.

    • MAX is the number of buckets;
    • FILL is the number of those buckets in use;
    • KEYS the number of keys in the hash.
    • In the empty case ARRAY will be null (even though MAX will be minimum 7), hence the 0 returned for the empty case.

    I think that the differences in moritz' benchmark come about because die is an uncommonly slow opcode--essentially a long jump--which completely swamps the conditional test and so skews the timing beyond recognition. 

    [0] Perl> for(2..6){ 
    %a =(); %b = 1.."1e$_"; 
    cmpthese -1, { 
        A=>q[ for(1 .. 10000){     if( %a ){ 1; } } ], 
        B=>q[ for(1 .. 10000){ unless( %b ){ 1; } } ] 
    } 
};;
    Rate    B    A
B 72.0/s   -- -90%
A  726/s 908%   --
    Rate    B    A
B 72.6/s   -- -90%
A  736/s 914%   --
    Rate    B    A
B 70.5/s   -- -91%
A  751/s 965%   --
    Rate    B    A
B 69.5/s   -- -91%
A  759/s 992%   --
    Rate    B    A
B 71.5/s   -- -90%
A  715/s 900%   --

[0] Perl> for(2..6){ 
    %a =(); %b = 1.."1e$_"; 
    cmpthese -1, { 
        A=>q[ for(1 .. 10000){ unless( %a ){ 1; } } ], 
        B=>q[ for(1 .. 10000){     if( %b ){ 1; } } ] 
    } 
};;
    Rate    B    A
B 75.9/s   -- -89%
A  709/s 834%   --
    Rate    B    A
B 73.7/s   -- -90%
A  730/s 890%   --
    Rate    B    A
B 72.7/s   -- -90%
A  747/s 928%   --
    Rate    B    A
B 71.5/s   -- -90%
A  725/s 914%   --
    Rate    B    A
B 70.6/s   -- -90%
A  708/s 903%   --

    [download]

    The difference between empty and full hashes is remarkably consistant--if surprisingly high--regardless of the magnitude of the hash.

    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.
[reply]
[d/l]

In Section Seekers of Perl Wisdom