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Recursively walk a hash to get to an element

by merlyn (Sage)
on Mar 30, 2005 at 19:26 UTC ( #443584=snippet: print w/ replies, xml ) Need Help??

Description: The question comes up frequently:
How do I start with %myhash, and a list of keys qw(bedrock flintstone fred), and get access to the value at $myhash{bedrock}{flintstone}{fred}?
Well, you call pointer_to_element, like:
my $ref = pointer_to_element(\%myhash, qw(bedrock flintstone fred)); $$ref = 42; # set it to 42 $$ref++; # increment it print $$ref; # print it!
sub pointer_to_element {
  require List::Util;
  return List::Util::reduce(sub { \($$a->{$b}) }, \shift, @_);
}
Comment on Recursively walk a hash to get to an element
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Re: Recursively walk a hash to get to an element
by princepawn (Parson) on Mar 31, 2005 at 00:36 UTC
    That is definitely one for Higher Order Perl, Second Edition.


    Carter's compass: I know I'm on the right track when by deleting something, I'm adding functionality
Re: Recursively walk a hash to get to an element
by tlm (Prior) on Mar 31, 2005 at 02:00 UTC

    There's an typo in the code; the code block in the reduce statement should be

    { \($$a->{$b}) }
    Also, when I tried the code as given (after fixing the above typo), I got the error:
    % perl rec_walk.pl Can't call method "List::Util::reduce" on unblessed reference at rec_w +alk.pl line 9.
    Here's the full code of the script that caused that error:
    use strict; my %myhash; $myhash{bedrock}{flintstone}{fred} = 3; my $ref = pointer_to_element(\%myhash, qw(bedrock flintstone fred)); sub pointer_to_element { require List::Util; return List::Util::reduce { \($$a->{$b}) } \shift, @_; }
    I couldn't figure out why the error, so I switched the code to
    use List::Util 'reduce'; sub pointer_to_element { return reduce { \($$a->{$b}) } \shift, @_; }
    and it worked great. Tres cool.

    the lowliest monk

      The reason of the error is that reduce uses a prototype not yet seen when the return statement is compiled. It's therefore interpreted as indirect object syntax (method { STATEMENTS } LIST), being do { STATEMENTS }->method(LIST) with direct syntax. The way to fix this is to call &reduce like List::Util::reduce(sub { ... }, LIST).

      ihb

      See perltoc if you don't know which perldoc to read!

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