HTML::Element::traverse - Online Linux Manual PageSection : 3pm
Updated : 2019-01-13
Source : perl v5.28.1
Note : User Contributed Perl Documentation

NAMEHTML::Element::traverse − discussion of HTML::Element's traverse method

VERSIONThis document describes version 5.07 of HTML::Element::traverse, released August 31, 2017 as part of HTML-Tree.

SYNOPSIS​ # $element−>traverse is unnecessary and obscure. ​ # Don't use it in new code.

DESCRIPTIONHTML::Element provides a method traverse that traverses the tree and calls user-specified callbacks for each node, in pre− or post-order. However, use of the method is quite superfluous: if you want to recursively visit every node in the tree, it's almost always simpler to write a subroutine does just that, than it is to bundle up the pre− and/or post-order code in callbacks for the traverse method.

EXAMPLESSuppose you want to traverse at/under a node $tree and give elements an 'id' attribute unless they already have one. You can use the traverse method: { ​ my $counter = 'x0000'; ​ $start_node−>traverse([ # Callbacks; ​ # pre−order callback: ​ sub { ​ my $x = $_[0]; ​ $x−>attr('id', $counter++) unless defined $x−>attr('id'); ​ return HTML::Element::OK; # keep traversing ​ }, ​ # post−order callback: ​ undef ​ ], ​ 1, # don't call the callbacks for text nodes ​ ); ​ } or you can just be simple and clear (and not have to understand the calling format for traverse) by writing a sub that traverses the tree by just calling itself: { ​ my $counter = 'x0000'; ​ sub give_id { ​ my $x = $_[0]; ​ $x−>attr('id', $counter++) unless defined $x−>attr('id'); ​ foreach my $c ($x−>content_list) { ​ give_id($c) if ref $c; # ignore text nodes ​ }}; ​ give_id($start_node); ​ } See, isn't that nice and clear? But, if you really need to know:

THE TRAVERSE METHODThe traverse() method is a general object-method for traversing a tree or subtree and calling user-specified callbacks. It accepts the following syntaxes: $h>traverse(\&callback) or $h>traverse(\&callback, $ignore_text) or $h>traverse( [\&pre_callback,\&post_callback] , $ignore_text) These all mean to traverse the element and all of its children. That is, this method starts at node $h, pre-order visits $h, traverses its children, and then will post-order visit $h. Visiting means that the callback routine is called, with these arguments: ​ $_[0] : the node (element or text segment), ​ $_[1] : a startflag, and ​ $_[2] : the depth If the $ignore_text parameter is given and true, then the pre-order call will not be happen for text content. The startflag is 1 when we enter a node (i.e., in pre-order calls) and 0 when we leave the node (in post-order calls). Note, however, that post-order calls don't happen for nodes that are text segments or are elements that are prototypically empty (like br, ​hr, etc.). If we visit text nodes (i.e., unless $ignore_text is given and true), then when text nodes are visited, we will also pass two extra arguments to the callback: ​ $_[3] : the element that's the parent ​ of this text node ​ $_[4] : the index of this text node ​ in its parent's content list Note that you can specify that the pre-order routine can be a different routine from the post-order one: ​ $h−>traverse( [\&pre_callback,\&post_callback], ...); You can also specify that no post-order calls are to be made, by providing a false value as the post-order routine: ​ $h−>traverse([ \&pre_callback,0 ], ...); And similarly for suppressing pre-order callbacks: ​ $h−>traverse([ 0,\&post_callback ], ...); Note that these two syntaxes specify the same operation: ​ $h−>traverse([\&foo,\&foo], ...); ​ $h−>traverse( \&foo , ...); The return values from calls to your pre− or post-order routines are significant, and are used to control recursion into the tree. These are the values you can return, listed in descending order of my estimation of their usefulness: HTML::Element::OK, 1, or any other true value ​...to keep on traversing. Note that HTML::Element::OK et al are constants. So if you're running under use strict (as I hope you are), and you say: ​return HTML::Element::PRUEN the compiler will flag this as an error (an unallowable bareword, specifically), whereas if you spell PRUNE correctly, the compiler will not complain. undef, 0, '0', '', or HTML::Element::PRUNE ​...to block traversing under the current element's content. (This is ignored if received from a post-order callback, since by then the recursion has already happened.) If this is returned by a pre-order callback, no post-order callback for the current node will happen. (Recall that if your callback exits with just return;, it is returning undef \*(-- at least in scalar context, and ​traverse always calls your callbacks in scalar context.) HTML::Element::ABORT ​...to abort the whole traversal immediately. This is often useful when you're looking for just the first node in the tree that meets some criterion of yours. HTML::Element::PRUNE_UP ​...to abort continued traversal into this node and its parent node. No post-order callback for the current or parent node will happen. HTML::Element::PRUNE_SOFTLY Like PRUNE, except that the post-order call for the current node is not blocked. Almost every task to do with extracting information from a tree can be expressed in terms of traverse operations (usually in only one pass, and usually paying attention to only pre-order, or to only post-order), or operations based on traversing. (In fact, many of the other methods in this class are basically calls to traverse() with particular arguments.) The source code for HTML::Element and HTML::TreeBuilder contain several examples of the use of the traverse method to gather information about the content of trees and subtrees. (Note: you should not change the structure of a tree while you are traversing it.) [End of documentation for the traverse() method]

Traversing with Recursive Anonymous RoutinesNow, if you've been reading ​Structure and Interpretation of Computer Programs too much, maybe you even want a recursive lambda. Go ahead: { ​ my $counter = 'x0000'; ​ my $give_id; ​ $give_id = sub { ​ my $x = $_[0]; ​ $x−>attr('id', $counter++) unless defined $x−>attr('id'); ​ foreach my $c ($x−>content_list) { ​ $give_id−>($c) if ref $c; # ignore text nodes ​ }}; ​ $give_id−>($start_node); ​ undef $give_id; ​ } It's a bit nutty, and it's still more concise than a call to the ​traverse method! It is left as an exercise to the reader to figure out how to do the same thing without using a $give_id symbol at all. It is also left as an exercise to the reader to figure out why I undefine $give_id, above; and why I could achieved the same effect with any of: ​ $give_id = 'I like pie!'; ​ # or... ​ $give_id = []; ​ # or even; ​ $give_id = sub { print "Mmmm pie!\n" }; But not: ​ $give_id = sub { print "I'm $give_id and I like pie!\n" }; ​ # nor... ​ $give_id = \$give_id; ​ # nor... ​ $give_id = { 'pie' => \$give_id, 'mode' => 'a la' };

Doing Recursive Things IterativelyNote that you may at times see an iterative implementation of pre-order traversal, like so: { ​ my @to_do = ($tree); # start−node ​ while(@to_do) { ​ my $this = shift @to_do; ​ ​ # "Visit" the node: ​ $this−>attr('id', $counter++) ​ unless defined $this−>attr('id'); ​ ​ unshift @to_do, grep ref $_, $this−>content_list; ​ # Put children on the stack −− they'll be visited next ​ }} This can under certain circumstances be more efficient than just a normal recursive routine, but at the cost of being rather obscure. It gains efficiency by avoiding the overhead of function-calling, but since there are several method dispatches however you do it (to ​attr and content_list), the overhead for a simple function call is insignificant.

Pruning and WhatnotThe traverse method does have the fairly neat features of the ABORT, PRUNE_UP and PRUNE_SOFTLY signals. None of these can be implemented totally straightforwardly with recursive routines, but it is quite possible. ABORT−like behavior can be implemented either with using non-local returning with eval/die: ​ my $died_on; # if you need to know where... ​ sub thing { ​ ... visits $_[0]... ​ ... maybe set $died_on to $_[0] and die "ABORT_TRAV" ... ​ ... else call thing($child) for each child... ​ ...any post−order visiting $_[0]... ​ } ​ eval { thing($node) }; ​ if($@) { ​ if($@ =~ m<^ABORT_TRAV>) { ​ ...it died (aborted) on $died_on... ​ } else { ​ die $@; # some REAL error happened ​ }} or you can just do it with flags: ​ my($abort_flag, $died_on); ​ sub thing { ​ ... visits $_[0]... ​ ... maybe set $abort_flag = 1; $died_on = $_[0]; return; ​ foreach my $c ($_[0]>content_list) { ​ thing($c); ​ return if $abort_flag; ​ } ​ ...any post−order visiting $_[0]... ​ return; ​ } ​ ​ $abort_flag = $died_on = undef; ​ thing($node); ​ ...if defined $abort_flag, it died on $died_on

SEE ALSOHTML::Element

AUTHORCurrent maintainers: • Christopher J. Madsen <perl AT cjmweb.net> • Jeff Fearn <jfearn AT cpan.org> Original HTML-Tree author: • Gisle Aas Former maintainers: • Sean M. Burke • Andy Lester • Pete Krawczyk <petek AT cpan.org> You can follow or contribute to HTML-Tree's development at <https://github.com/kentfredric/HTML−Tree>.

COPYRIGHTCopyright 2000,2001 Sean M. Burke
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