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I'm surprised that returning a list of keys is faster than returning a hashref and then getting its keys. A dereference requires more operations, but should it be that much?

A good chunk of the time is spent in building the hash, so I added a baseline sub to measure it. I also reduced the number of iterations so it would run in reasonable time on my machine. And some minor formatting of results.

This is perl 5, version 28, subversion 0 (v5.28.0) built for MSWin32-x +64-multi-thread
# Adapted from use strict; use warnings; use Benchmark; use Data::Dumper; # Note: You can increment a string in Perl! WOW! # As long as you don't use the string in a numeric context! # $string++ is quite different than $string +=1 # # Below, this is used to make a bunch of unique hash keys # I don't think that the fact that they are sequential in # an alphabetic sense makes much difference in the generated # hash table because of the way that the Perl hash algorithm # works. sub baseline { # create a large hash but return nothing my $string = "ABCDEFGHIJ"; my %hash = map{$string++ => 1}(1..100000); return; } sub return_hash { # create a large hash and return that entire hash as list my $string = "ABCDEFGHIJ"; my %hash = map{$string++ => 1}(1..100000); return %hash; } sub return_hash_ref { # create a large hash and return a ref to that hash my $string = "ABCDEFGHIJ"; my %hash = map{$string++ => 1}(1..100000); return \%hash; } sub return_just_keys { # create a large hash and return just the keys of that hash my $string = "ABCDEFGHIJ"; my %hash = map{$string++ => 1}(1..100000); return keys %hash; } timethese(200, { '1)Keys of Hash via list ' => 'my @keys = keys %{{return_hash()}} +', '2)Keys of Local Hash copy' => 'my %hash2 = return_hash(); my @key +s = keys %hash2;', '3)Keys of local Hash Ref ' => 'my $href = return_hash_ref(); my @ +keys = keys %$href;', '4)Just the returned keys ' => 'my @keys = return_just_keys()', '5)Baseline ' => 'my $res = baseline()', }); __END__ Benchmark: timing 200 iterations of 1)Keys of Hash via list , 2)Keys +of Local Hash copy, 3)Keys of local Hash Ref , 4)Just the returned ke +ys , 5)Baseline ... 1)Keys of Hash via list : 50 wallclock secs (49.44 usr + 1.52 sys = +50.95 CPU) @ 3.93/s (n=200) 2)Keys of Local Hash copy: 50 wallclock secs (49.98 usr + 0.38 sys = +50.36 CPU) @ 3.97/s (n=200) 3)Keys of local Hash Ref : 35 wallclock secs (34.75 usr + 0.34 sys = +35.09 CPU) @ 5.70/s (n=200) 4)Just the returned keys : 33 wallclock secs (32.51 usr + 0.16 sys = +32.67 CPU) @ 6.12/s (n=200) 5)Baseline : 25 wallclock secs (24.86 usr + 0.20 sys = +25.06 CPU) @ 7.98/s (n=200)

In reply to Re^2: Using 'keys' on a list by swl
in thread Using 'keys' on a list by lammey

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