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Re: Benifits of using hash rather than number of variables?

by DrHyde (Prior)
on Jul 20, 2011 at 10:11 UTC ( #915625=note: print w/ replies, xml ) Need Help??


in reply to Benifits of using hash rather than number of variables?

The three big advantages of passing hashes / hashrefs to your subroutines are:

  1. you have named parameters inside the subroutine, and you don't have to remember the order of arguments outside the subroutine;
  2. much easier to handle optional arguments, and to calculate which arguments to pass at runtime;
  3. easy to build up that hash a little bit at a time;

The big disadvantage of using hashes is that you lose some strictness checking. use strict will usually tell you if you mis-type a variable name, but it can't tell you if you mis-type a hash key - instead, if you mis-type a key name when reading the hash, it'll just auto-vivify. This sort of bug can be a real pain in the arse to track down.

Thankfully, there's a solution to this problem on the CPAN. Tie::Hash::Vivify is meant to let you provide "a hash where if you read a key that doesn't exist, it will call a code reference to fill that slot with a value", but it can of course be used thus:

use Tie::Hash::Vivify; use Carp qw(confess); use Data::Dumper; ... my $hashref = Tie::Hash::Vivify->new(sub { confess("No auto-vivifying!\n".Dumper(\@_)) });


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Re^2: Benifits of using hash rather than number of variables?
by duelafn (Priest) on Jul 20, 2011 at 14:45 UTC

    See also Hash::Util which is in perl core since 5.8.0

    use Hash::Util qw( lock_keys ); my %hash = (foo => 42, bar => 23); lock_keys(%hash); say $hash{bae}; # Boom! "Attempt to access disallowed key 'bae' in a + restricted hash..."

    Good Day,
        Dean

      That doesn't really work for named parameters, specially not if some parameters are optional. Unless you're going to do something unwieldy like:
      sub whatever { unless (@_) { my %params = (key1 => undef, key2 => undef, key3 => undef,); lock_keys %params; return \%params; } my %params = %{+shift}; ... sub body using named parameters ... } my $whatever_params = whatever(); $whatever_params->{key1} = $value1; $whatever_params->{key2} = $value2; $whatever_params->{key3} = $value3; whatever $whatever_params;

        It will work fine for sub parameters. You can provide a list of keys as a second parameter to lock_keys:

        sub whatever { my %params = @_; lock_keys %params, qw/ key1 key2 key3 /; say "Got key1" if $params{key1}; # ok say "Got key2" if $params{key2}; # ok say "Got key3" if $params{keye}; # Oops - BOOM! } whatever key2 => $value2; whatever keyq => $value1; # Hash has key 'keyq' which is not in the ne +w key set

        For example, I use the following for parsing command-line options in my scripts:

        use Getopt::Long qw/:config bundling/; use Hash::Util qw/ lock_keys /; our %OPT; my @OPT_SPEC = qw/ help|h version noact|no-act|dry-run DEBUG output|o= +s /; GetOptions \%OPT, @OPT_SPEC or ($OPT{help} = 1); lock_keys(%OPT, map /^(\w+)/, @OPT_SPEC);

        Update 2: A diligent Anonymous monk actually tested this code (actually executing proposed code - what a concept) and pointed out that the initial lock_keys suggestion is sufficient. Updated the comments in the code above to reflect this.

        Update: Add example where lock_keys is not quite sufficient... One could solve this in a couple ways without changing how whatever is called: Anonymous monk make all this unnecessary

        # Option 1: use Params::Validate # Option 2: sub whatever { my @param_keys = qw/ key1 key2 key3 /; my %params = @_; $params{$_} //= undef for @param_keys; die "Invalid usage" if @param_keys != (keys %params); lock_keys %params; # ... } # Option 3: sub whatever { my %expected_params = map +($_,1), qw/ key1 key2 key3 /; my %params = @_; for (keys %params) { die "whatever: Unexpected named parameter $_" unless exists $e +xpected_params{$_}; } lock_keys %params, keys %expected_params; # ... }

        Good Day,
            Dean

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