in reply to Recursively walk a hash to get to an element

The following is the same solution but faster since it eliminates all those slow sub calls:

sub DiveRef { my $p = \shift; $p = \( $$p->{$_} ) for @_; $p } my %myhash; my $ref = DiveRef(\%myhash, qw( bedrock flintstone fred )); -or-- my $myhash; my $ref = DiveRef($myhash, qw( bedrock flintstone fred )); $$ref = 42; # set it to 42 $$ref++; # increment it print $$ref; # print it!

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Re^2: Recursively walk a hash to get to an element
by LanX (Cardinal) on Apr 04, 2021 at 19:20 UTC
    Adding some syntactic sugar with prototype and lvalue (not sure when the + prototype was introduced UPDATE: 5.14.0 code corrected)

    Checking the ref-type would help diving arrays too. (but would probably reinvent Data::Diver )

    use v5.14; use warnings; use Test::More; sub dive(+;@) :lvalue { my $h = \ shift; $h = \($$h->{$_}) for @_; $$h; } my %hsh; my $h_ref = \%hsh; my $val = $hsh{a}{b}{c} = 3; my $val2 = 42; my @path = qw/a b c/; my $test; ;;;;;;;;;; $test = "Prototypes"; is( dive(%hsh => @path) , $val, "$test accepts \%list form"); is( dive($h_ref=> @path) , $val, "$test accepts \$ref form"); ;;;;;;;;;; $test = "assign directly to lvalue"; dive(%hsh=> @path) = $val2; is( $hsh{a}{b}{c}, $val2, $test); ;;;;;;;;;; $test = "increment in place"; dive( %hsh=> @path )++; is( $hsh{a}{b}{c}, ++$val2, $test); ;;;;;;;;;; $test = "grab scalar reference"; my $s_ref = \ dive(%hsh, @path); $$s_ref++; is( $hsh{a}{b}{c}, ++$val2, $test); ;;;;;;;;;; $test = "aliasing"; for my $alias ( dive(%hsh, @path) ) { $alias++; } is( $hsh{a}{b}{c}, ++$val2, $test); done_testing;
    ok 1 - Prototypes accepts %list form ok 2 - Prototypes accepts $ref form ok 3 - assign directly to lvalue ok 4 - increment in place ok 5 - grab scalar reference ok 6 - aliasing 1..6

    UPDATE: cosmetic corrections

    Cheers Rolf
    (addicted to the Perl Programming Language :)
    Wikisyntax for the Monastery

      The code I posted was based on code that did use :lvalue, but I adapted it to match the OP.


      I strongly recommend using an existing solution such as Data::Diver.

      use Data::Diver qw( DiveVal ); my @keys = map "level$_", 1 .. 3; my $branch = {}; DiveVal($branch, map \$_, @keys) = 'leaf'; -or- my %branch; DiveVal(\%branch, map \$_, @keys) = 'leaf';

      Obviously, it can be done without module too.

      sub DiveVal :lvalue { my $p = \shift; $p = \( $$p->{$_} ) for @_; $$p } my @keys = map "level$_", 1 .. 3; my $branch; DiveVal($branch, @keys) = 'leaf'; -or- my %branch; DiveVal(\%branch, @keys) = 'leaf';

      How my DiveVal works:

      Pre-loop: $p references $branch After loop pass 0: $p references $branch->{level1} After loop pass 1: $p references $branch->{level1}{level2} After loop pass 2: $p references $branch->{level1}{level2}{level3} Returned: $branch->{level1}{level2}{level3}

      The extra level of indirection has many benefits.

      • It removes the need to treat the last key specially.
      • It removes the need to create the hash before it's dereferenced.
      • It removes the need for the root to be a reference to a hash. Instead, any scalar can be the root, even an undefined one.
      • It makes it easy to extend DiveVal to support mixed array/hash structures.

      Being fundamentally the same solution, the explanation applies to Merlyn's code too. Just substitute $a for $p, and it's a reference to the final scalar that's returned instead of the scalar itself as an lvalue.