Additional hash lookup using 'exists'?

问题:

I sometimes access a hash like this:

if(exists $ids{$name}){
    $id = $ids{$name};
}

Is that good practice? I'm a bit concerned that it contains two lookups where really one should be done. Is there a better way to check the existence and assign the value?

回答1:

By checking with exists, you prevent autovivification. See Autovivification : What is it and why do I care?.

UPDATE: As trendels points out below, autovivification does not come into play in the example you posted. I am assuming that the actual code involves multi-level hashes.

Here is an illustration:

#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

my (%hash, $x);

if ( exists $hash{test}->{vivify} ) {
    $x = $hash{test}->{vivify}->{now};
}

print Dumper \%hash;

$x = $hash{test}->{vivify}->{now};

print Dumper \%hash;

__END__


C:\Temp> t
$VAR1 = {
    'test' => {}
};
$VAR1 = {
    'test' => {
        'vivify' => {}
    }
};

回答2:

You could use apply Hash::Util's lock_keys to the hash. Then perform your assignments within an eval.

#!/usr/bin/perl
use Hash::Util qw/lock_keys/;

my %a = (
    1 => 'one',
    2 => 'two'
);

lock_keys(%a);

eval {$val = $a{2}};     # this assignment completes
eval {$val = $a{3}};     # this assignment aborts
print "val=$val\n";      # has value 'two'

回答3:

You can do it with one lookup like this:

$tmp = $ids{$name};
$id = $tmp if (defined $tmp);

However, I wouldn't bother unless I saw that that was a bottleneck

回答4:

if it is not a multi-level hash you can do this:

$id = $ids{$name} || 'foo';

or if $id already has a value:

$id ||= $ids{$name};

where 'foo' is a default or fall-through value. If it is a multi-level hash you would use 'exists' to avoid the autovivification discussed earlier in the thread or not use it if autovivification is not going to be a problem.

回答5:

If I want high performance I'm used to write this idiom when want create hash as set:

my %h;
for my $key (@some_vals) {
  ...
  $h{$key} = undef unless exists $h{$key};
  ...
}

return keys %h;

This code is little bit faster than commonly used $h{$key}++. exists avoids useless assignment and undef avoids allocation for value. Best answer for you is: Benchmark it! I guess that exists $ids{$name} is little bit faster than $id=$ids{$name} and if you have big miss ratio your version with exists can be faster than assignment and test after.

For example if I want fast sets intersection I would wrote something like this.

sub intersect {
  my $h;
  @$h{@{shift()}} = ();
  my $i;
  for (@_) {
    return unless %$h;
    $i = {};
    @$i{grep exists $h->{$_}, @$_} = ();
    $h = $i;
  }
  return keys %$h;
}

回答6:

performance is not important in this case see "Devel::NYTProf". But to answer your question:

if the value in the hash does not exists, "exists" is very fast

if(exists $ids{$name}){
    $id = $ids{$name};
}

but if it does exists a second lookup is done. if the value is likely to exists than making only one look up will be faster

$id = $ids{$name};
if($id){
    #....
}

see this littel benchmark from a perl mailing list.

#!/usr/bin/perl -w
use strict;
use Benchmark qw( timethese );

use vars qw( %hash );
@hash{ 'A' .. 'Z', 'a' .. 'z' } = (1) x 52;

my $key = 'xx';
timethese 10000000, {
        'defined' => sub {
                if (defined $hash{$key}) { my $x = $hash{$key}; return $x; };
                return 0;
        },
        'defined_smart' => sub {
                my $x = $hash{$key};
                if (defined $x) {
                        return $x;
                };
                return 0;
        },
        'exists' => sub {
                if (exists $hash{$key}) { my $x = $hash{$key}; return $x; };
                return 0;
        },
        'as is' => sub {
                if ($hash{$key}) { my $x = $hash{$key}; return $x; };
                return 0;
        },
        'as is_smart' => sub {
                my $x = $hash{$key};
                if ($x) { return $x; };
                return 0;
        },

};

using a key('xx') that does not exists shows that 'exists' is the winner.

Benchmark: timing 10000000 iterations of as is, as is_smart, defined, defined_smart, exists...
     as is:  1 wallclock secs ( 1.52 usr +  0.00 sys =  1.52 CPU) @ 6578947.37/s (n=10000000)
as is_smart:  3 wallclock secs ( 2.67 usr +  0.00 sys =  2.67 CPU) @ 3745318.35/s (n=10000000)
   defined:  3 wallclock secs ( 1.53 usr +  0.00 sys =  1.53 CPU) @ 6535947.71/s (n=10000000)
defined_smart:  3 wallclock secs ( 2.17 usr +  0.00 sys =  2.17 CPU) @ 4608294.93/s (n=10000000)
    exists:  1 wallclock secs ( 1.33 usr +  0.00 sys =  1.33 CPU) @ 7518796.99/s (n=10000000)

using a key('x') that does exists shows that 'as is_smart' is the winner.

Benchmark: timing 10000000 iterations of as is, as is_smart, defined, defined_smart, exists...
     as is:  3 wallclock secs ( 2.76 usr +  0.00 sys =  2.76 CPU) @ 3623188.41/s (n=10000000)
as is_smart:  3 wallclock secs ( 1.81 usr +  0.00 sys =  1.81 CPU) @ 5524861.88/s (n=10000000)
   defined:  3 wallclock secs ( 3.42 usr +  0.00 sys =  3.42 CPU) @ 2923976.61/s (n=10000000)
defined_smart:  2 wallclock secs ( 2.32 usr +  0.00 sys =  2.32 CPU) @ 4310344.83/s (n=10000000)
    exists:  3 wallclock secs ( 2.83 usr +  0.00 sys =  2.83 CPU) @ 3533568.90/s (n=10000000)