Limits to XPath implementaiton in PHP5

3 Message(s) by 2 Author(s) originally posted in php general

From: Weston

Date: Monday, October 22, 2007

Poking around with XPath using SimpleXML, it looks like there are at
least a few reasonably common XPath operator

<programming> A symbol used as a {function}, with {infix
	syntax} if it has two arguments (e.g. "+") or {prefix syntax} if
	it has only one (e.g. {Boolean} NOT).  Many languages use
	operators for built-in functions such as arithmetic and logic.

	(1995-04-30)

s and predicates that
are not supported. I'd like to check my observations against other
people's experience, and find out if I'm missing something.

Here's what it looks like to me in PHP 5.2.0:

* the count() predicate seems to fail quietly (no error

1. A discrepancy between a computed, observed, or measured
	value or condition and the true, specified, or theoretically
	correct value or condition.

	2. <programming> A mental mistake made by a programmer that
	may result in a program {fault}.

	3. (verb) What a program does when it stops as result of a
	programming error.

	(2000-03-28)

or warning,
you just receive a false value from the xpath method

<programming> The name given in {Smalltalk} and other
	{object-oriented languages} to a procedure or routine
	associated with one or more {classes}.  An {object} of a
	certain class knows how to perform actions, e.g. printing
	itself or creating a new instance of itself, rather than the
	function (e.g. printing) knowing how to handle different types
	of object.

	Different classes may define methods with the same name
	(i.e. methods may be {polymorphic}).  The term "method" is used
	both for a named operation, e.g. "PRINT" and also for the code
	which a specific class provides to perform tha
	t operation.

	Most methods operate on objects that are instances of a
	certain class.  Some object-oriented languages call these
	"object methods" to distinguish then from "{class methods}".

	In {Smalltalk}, a method is defined by giving its name,
	documentation, temporary local variables and a sequence of
	expressions separated by "."s.

	(2000-03-22)

rather than an
array)
* the "|" (union) operator seems to also fail quietly by returning an
empty array
* the "union" operator seems to cause an error
* the "intersect" operator also causes an error (and needless to say,
with | and count() not working, using something like the Kaysian
technique to get the intersection won't work)

Does this accurately capture the current

<electronics> The quantity of {charge} per unit time, measured
	in Amperes (Amps, A).  By historical convention, the sign of
	current is positive for currents flowing from positive to
	negative {potential}, but experience indicates that electrons
	are negatively charged and flow in the opposite direction.

	(1995-10-05)

state

<storage, architecture, jargon, theory> How something is; its
	configuration, attributes, condition, or information content.
	The state of a system is usually temporary (i.e. it changes
	with time) and volatile (i.e. it will be lost or reset to some
	initial state if the system is switched off).

	A state may be considered to be a point in some {space} of all
	possible states.  A simple example is a light, which is either
	on or off.  A complex example is the electrical activation in
	a human brain while solving a problem.

	In computing and related fields, states, as in the light
	example, are often modelled as being {discrete} (rather than
	continuous) and the transition from one state to another is
	considered to be instantaneous.  Another (related) property of
	a system is the number of possible states it may exhibit.
	This may be finite or infinite.  A common model for a system
	with a finite number of discrete state is a {finite state
	machine}.

	[{Jargon File}]

	(1996-10-13)

of things?

If so, are the bulk of developers simply not using these features, or
are they instead writing around the missing features in PHP rather
than trying to get XPath to do the work?

Or is there a better option?

From: Jeremy

Date: Monday, October 22, 2007

wrote in message:

Poking around with XPath using SimpleXML, it looks like there are at
least a few reasonably common XPath operators and predicates that
are not supported. I'd like to check my observations against other
people's experience, and find out if I'm missing something.
Here's what it looks like to me in PHP 5.2.0:
* the count() predicate seems to fail quietly (no error or warning,
you just receive a false value from the xpath method rather than an
array)
* the "|" (union) operator seems to also fail quietly by returning an
empty array
* the "union" operator seems to cause an error
* the "intersect" operator also causes an error (and needless to say,
with | and count() not working, using something like the Kaysian
technique to get the intersection won't work)
Does this accurately capture the current state of things?
If so, are the bulk of developers simply not using these features, or
are they instead writing around the missing features in PHP rather
than trying to get XPath to do the work?
Or is there a better option?

AFAIK, the various PHP API

<programming> (API, or "application programming interface")
	The interface (calling conventions) by which an {application
	program} accesses {operating system} and other services.  An
	API is defined at {source code} level and provides a level of
	{abstraction} between the application and the {kernel} (or
	other privileged utilities) to ensure the {portability} of the
	code.

	An API can also provide an interface between a {high level
	language} and lower level utilities and services which were
	written without consideration for the {calling conventions}
	supported by compiled languages.  In this case, the API's main
	task may be the translation of parameter lists from one format
	to another and the interpretation of {call-by-value} and
	{call-by-reference} arguments in one or both directions.

	(1995-02-15)

's (SimpleXML, DOM

<hypertext, language, World-Wide Web> A {W3C} specification
	for {application program interfaces} for accessing the content
	of {HTML} and {XML} documents.

	{Home (http://www.w3.org/DOM/)}.

	(1999-12-14)

) use libxml as a backend,
without much logic

1. <philosophy, mathematics> A branch of philosophy and
	mathematics that deals with the formal principles, methods and
	criteria of validity of {inference}, reasoning and
	{knowledge}.

	Logic is concerned with what is true and how we can know
	whether something is true.  This involves the formalisation of
	logical arguments and {proofs} in terms of symbols
	representing {propositions} and {logical connectives}.  The
	meanings of these logical connectives are expressed by a set
	of rules which are assumed to be self-evident.

	{Boolean algebra} deals with the basic operations of truth
	values: AND, OR, NOT and combinations thereof.  {Predicate
	logic} extends this with existential and universal
	{quantifiers} and symbols standing for {predicates} which may
	depend on variables.  The rules of {natural deduction}
	describe how we may proceed from valid premises to valid
	conclusions, where the premises and conclusions are
	expressions in {predicate logic}.

	Symbolic logic uses a {meta-language} concerned with truth,
	which may or may not have a corresponding expression in the
	world of objects called existance.  In symbolic logic,
	arguments and {proofs} are made in terms of symbols
	representing {propositions} and {logical connectives}.  The
	meanings of these begin with a set of rules or {primitives}
	which are assumed to be self-evident.  Fortunately, even from
	vague primitives, functions can be defined with precise
	meaning.

	{Boolean logic} deals with the basic operations of {truth
	values}: AND, OR, NOT and combinations thereof.  {Predicate
	logic} extends this with {existential quantifiers} and
	{universal quantifiers} which introduce {bound variables}
	ranging over {finite} sets; the {predicate} itself takes on
	only the values true and false.  Deduction describes how we
	may proceed from valid {premises} to valid conclusions, where
	these are expressions in {predicate logic}.

	Carnap used the phrase "rational reconstruction" to describe
	the logical analysis of thought.  Thus logic is less concerned
	with how thought does proceed, which is considered the realm
	of psychology, and more with how it should proceed to discover
	truth.  It is the touchstone of the results of thinking, but
	neither its regulator nor a motive for its practice.

	See also fuzzy logic, logic programming, arithmetic and logic unit,
	first-order logic,

	See also {Boolean logic}, {fuzzy logic}, {logic programming},
	{first-order logic}, {logic bomb}, {combinatory logic},
	{higher-order logic}, {intuitionistic logic}, {equational
	logic}, {modal logic}, {linear logic}, {paradox}.

	2. <electronics> {Boolean} logic circuits.

	See also {arithmetic and logic unit}, {asynchronous logic},
	{TTL}.

	(1995-03-17)

in between. So if the XPath operators are supported
by libxml, they ought to be supported by PHP. If they're not supported
by libxml, that'd really surprise me.

That said, try using the DOM API (application programming

1. The art of debugging a blank sheet of paper (or, in these
	days of on-line editing, the art of debugging an empty file).

	2. A pastime similar to banging one's head against a wall, but
	with fewer opportunities for reward.

	3. The most fun you can have with your clothes on (although
	clothes are not mandatory).

	[{Jargon File}]

	(2003-02-12)

interface)instead of SimpleXML and see if you
have better results. Then, try peeking into the libxml error stack

<programming> (See below for synonyms) A data structure for
	storing items which are to be accessed in last-in first-out
	order.

	The operations on a stack are to create a new stack, to "push"
	a new item onto the top of a stack and to "pop" the top item
	off.  Error conditions are raised by attempts to pop an empty
	stack or to push an item onto a stack which has no room for
	further items (because of its implementation).

	Most processors include support for stacks in their
	{instruction set architectures}.  Perhaps the most common use
	of stacks is to store {subroutine} arguments and return
	addresses.  This is usually supported at the {machine code}
	level either directly by "jump to subroutine" and "return from
	subroutine" instructions or by {auto-increment} and
	auto-decrement {addressing modes}, or both.  These allow a
	contiguous area of memory to be set aside for use as a stack
	and use either a special-purpose {register} or a general
	purpose register, chosen by the user, as a {stack pointer}.

	The use of a stack allows subroutines to be {recursive} since
	each call can have its own calling context, represented by a
	stack frame or {activation record}.  There are many other
	uses.  The programming language {Forth} uses a data stack in
	place of variables when possible.

	Although a stack may be considered an {object} by users,
	implementations of the object and its access details differ.
	For example, a stack may be either ascending (top of stack is
	at highest address) or descending.  It may also be "full" (the
	stack pointer points at the top of stack) or "empty" (the
	stack pointer points just past the top of stack, where the
	next element would be pushed).  The full/empty terminology is
	used in the {Acorn Risc Machine} and possibly elsewhere.

	In a list-based or {functional language}, a stack might be
	implemented as a {linked list} where a new stack is an empty
	list, push adds a new element to the head of the list and pop
	splits the list into its head (the popped element) and tail
	(the stack in its modified form).

	At {MIT}, {pdl} used to be a more common synonym for stack,
	and this may still be true.  {Knuth} ("The Art of Computer
	Programming", second edition, vol. 1, p. 236) says:

	  Many people who realised the importance of stacks and queues
	  independently have given other names to these structures:
	  stacks have been called push-down lists, reversion storages,
	  cellars, dumps, nesting stores, piles, last-in first-out
	  ("LIFO") lists, and even yo-yo lists!

	[{Jargon File}]

	(1995-04-10)

directly using the libxml functions:

http://us.php.net/manual/en/ref.libxml.php

Jeremy

From: Weston

Date: Thursday, October 25, 2007

wrote in message:

That said, try using the DOM API (application programming interface)instead of SimpleXML and see if you
have better results.

There are indeed some different results. Here's a basic rundown:

count() predicate: succesful under DOM, fails quietly under SimpleXML
"|" (union) operator: succesful under DOM, not sure under SimpleXML
"union" operator: fails with error under both
"intersect" operator: fails with error under both
kaysian intersection technique: succesful under DOM, fails under
SimpleXML

Then, try peeking into the libxml error stack
directly using the libxml functions:
http://us.php.net/manual/en/ref.libxml.php

I'll have to poke

The {BASIC} command to write a value to an absolute address.

	See {peek}.

	[{Jargon File}]

	(1995-01-31)

at that further, but at this point

1. <unit, text> (Sometimes abbreviated "pt") The unit of
	length used in {typography} to specify text character height,
	{rule} width, and other small measurements.

	There are six slightly different definitions: {Truchet point},
	{Didot point}, {ATA point}, {TeX point}, {Postscript point},
	and {IN point}.

	In Europe, the most commonly used is Didot and in the US, the
	formerly standard ATA point has essentially been replaced by
	the PostScript point due to the demise of traditional
	typesetting systems and rise of desktop computer based systems
	running software such as {QuarkXPress}, {Adobe InDesign} and
	{Adobe Pagemaker}.

	There are 20 {twips} in a point and 12 points in a {pica}
	(known as a "Cicero" in the Didot system).

	{Different point systems
	(http://www.vakcer.com/oberon/dtp/fonts/point.htm)}.

	(2004-12-23)

	2. <hardware> To move a {pointing device} so that the
	on-screen pointer is positioned over a certain object on the
	screen such as a {button} in a {graphical user interface}.  In
	most {window systems} it is then necessary to {click} a
	(physical) button on the pointing device to activate or select
	the object.  In some systems, just pointing to an object is
	known as "mouse-over" {event} which may cause some help text
	(called a "tool tip" in {Windows}) to be displayed.

	(2001-05-21)

it does look like
both APIs can not simply be handing off to libxml.

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