xml2: xml_find_all – R documentation

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xml_find_all

Find nodes that match an xpath expression.

Description

Xpath is like regular expressions for trees - it's worth learning if you're trying to extract nodes from arbitrary locations in a document. Use xml_find_all to find all matches - if there's no match you'll get an empty result. Use xml_find_first to find a specific match - if there's no match you'll get an xml_missing node.

Usage

xml_find_all(x, xpath, ns = xml_ns(x))

xml_find_first(x, xpath, ns = xml_ns(x))

xml_find_num(x, xpath, ns = xml_ns(x))

xml_find_chr(x, xpath, ns = xml_ns(x))

xml_find_lgl(x, xpath, ns = xml_ns(x))

Arguments

`x`	A document, node, or node set.
`xpath`	A string containing a xpath (1.0) expression.
`ns`	Optionally, a named vector giving prefix-url pairs, as produced by `xml_ns()`. If provided, all names will be explicitly qualified with the ns prefix, i.e. if the element `bar` is defined in namespace `foo`, it will be called `foo:bar`. (And similarly for attributes). Default namespaces must be given an explicit name. The ns is ignored when using `xml_name<-()` and `xml_set_name()`.

Value

xml_find_all always returns a nodeset: if there are no matches the nodeset will be empty. The result will always be unique; repeated nodes are automatically de-duplicated.

xml_find_first returns a node if applied to a node, and a nodeset if applied to a nodeset. The output is always the same size as the input. If there are no matches, xml_find_first will return a missing node; if there are multiple matches, it will return the first only.

xml_find_num, xml_find_chr, xml_find_lgl return numeric, character and logical results respectively.

Deprecated functions

xml_find_one() has been deprecated. Instead use xml_find_first().

Examples

x <- read_xml("<foo><bar><baz/></bar><baz/></foo>")
xml_find_all(x, ".//baz")
xml_path(xml_find_all(x, ".//baz"))

# Note the difference between .// and //
# //  finds anywhere in the document (ignoring the current node)
# .// finds anywhere beneath the current node
(bar <- xml_find_all(x, ".//bar"))
xml_find_all(bar, ".//baz")
xml_find_all(bar, "//baz")

# Find all vs find one -----------------------------------------------------
x <- read_xml("<body>
  <p>Some <b>text</b>.</p>
  <p>Some <b>other</b> <b>text</b>.</p>
  <p>No bold here!</p>
</body>")
para <- xml_find_all(x, ".//p")

# If you apply xml_find_all to a nodeset, it finds all matches,
# de-duplicates them, and returns as a single list. This means you
# never know how many results you'll get
xml_find_all(para, ".//b")

# xml_find_first only returns the first match per input node. If there are 0
# matches it will return a missing node
xml_find_first(para, ".//b")
xml_text(xml_find_first(para, ".//b"))

# Namespaces ---------------------------------------------------------------
# If the document uses namespaces, you'll need use xml_ns to form
# a unique mapping between full namespace url and a short prefix
x <- read_xml('
 <root xmlns:f = "http://foo.com" xmlns:g = "http://bar.com">
   <f:doc><g:baz /></f:doc>
   <f:doc><g:baz /></f:doc>
 </root>
')
xml_find_all(x, ".//f:doc")
xml_find_all(x, ".//f:doc", xml_ns(x))

xml2

Parse XML

v1.3.2

GPL (>= 2)

Authors

Hadley Wickham [aut], Jim Hester [aut, cre], Jeroen Ooms [aut], RStudio [cph], R Foundation [ctb] (Copy of R-project homepage cached as example)

Initial release