Modifying existing XML can be done in xml2 by using the replacement
functions of the accessors. They all have methods for both individual
xml_node objects as well as xml_nodeset
objects. If a vector of values is provided it is applied piecewise over
the nodeset, otherwise the value is recycled.
Text modification only happens on text nodes. If a given node has
more than one text node only the first will be affected. If you want to
modify additional text nodes you need to select them explicitly with
/text().
x <- read_xml("<p>This is some <b>text</b>. This is more.</p>")
xml_text(x)
#> [1] "This is some text. This is more."
xml_text(x) <- "This is some other text."
xml_text(x)
#> [1] "This is some other text.text. This is more."
# You can avoid this by explicitly selecting the text node.
x <- read_xml("<p>This is some text. This is <b>bold!</b></p>")
text_only <- xml_find_all(x, "//text()")
xml_text(text_only) <- c("This is some other text. ", "Still bold!")
xml_text(x)
#> [1] "This is some other text. Still bold!"
xml_structure(x)
#> <p>
#> {text}
#> <b>
#> {text}Attributes and namespace definitions are modified one at a time with
xml_attr() or all at once with xml_attrs(). In
both cases using NULL as the value will remove the
attribute completely.
x <- read_xml("<a href='invalid!'>xml2</a>")
xml_attr(x, "href")
#> [1] "invalid!"
xml_attr(x, "href") <- "https://github.com/r-lib/xml2"
xml_attr(x, "href")
#> [1] "https://github.com/r-lib/xml2"
xml_attrs(x) <- c(id = "xml2", href = "https://github.com/r-lib/xml2")
xml_attrs(x)
#> href id
#> "https://github.com/r-lib/xml2" "xml2"
x
#> {xml_document}
#> <a href="https://github.com/r-lib/xml2" id="xml2">
xml_attrs(x) <- NULL
x
#> {xml_document}
#> <a>
# Namespaces are added with as a xmlns or xmlns:prefix attribute
xml_attr(x, "xmlns") <- "http://foo"
x
#> {xml_document}
#> <a xmlns="http://foo">
xml_attr(x, "xmlns:bar") <- "http://bar"
x
#> {xml_document}
#> <a xmlns="http://foo" xmlns:bar="http://bar">All of these functions have a .copy argument. If this is
set to FALSE they will remove the new node from its
location before inserting it into the new location. Otherwise they make
a copy of the node before insertion.
x <- read_xml("<parent><child>1</child><child>2<child>3</child></child></parent>")
children <- xml_children(x)
t1 <- children[[1]]
t2 <- children[[2]]
t3 <- xml_children(children[[2]])[[1]]
xml_replace(t1, t3)
#> {xml_node}
#> <child>
x
#> {xml_document}
#> <parent>
#> [1] <child>3</child>
#> [2] <child>2<child>3</child></child>x <- read_xml("<parent><child>1</child><child>2<child>3</child></child></parent>")
children <- xml_children(x)
t1 <- children[[1]]
t2 <- children[[2]]
t3 <- xml_children(children[[2]])[[1]]
xml_add_sibling(t1, t3)
x
#> {xml_document}
#> <parent>
#> [1] <child>1</child>
#> [2] <child>3</child>
#> [3] <child>2<child>3</child></child>
xml_add_sibling(t3, t1, where = "before")
x
#> {xml_document}
#> <parent>
#> [1] <child>1</child>
#> [2] <child>3</child>
#> [3] <child>2<child>3</child><child>1</child></child>x <- read_xml("<parent><child>1</child><child>2<child>3</child></child></parent>")
children <- xml_children(x)
t1 <- children[[1]]
t2 <- children[[2]]
t3 <- xml_children(children[[2]])[[1]]
xml_add_child(t1, t3)
x
#> {xml_document}
#> <parent>
#> [1] <child>1<child>3</child></child>
#> [2] <child>2<child>3</child></child>
xml_add_child(t1, read_xml("<test/>"))
x
#> {xml_document}
#> <parent>
#> [1] <child>1<child>3</child><test/></child>
#> [2] <child>2<child>3</child></child>The xml_remove() can be used to remove a node (and its
children) from a tree. The default behavior is to unlink the node from
the tree, but does not free the memory for the node, so R
objects pointing to the node are still valid.
This allows code like the following to work without crashing R
x <- read_xml("<foo><bar><baz/></bar></foo>")
x1 <- x %>%
xml_children() %>%
.[[1]]
x2 <- x1 %>%
xml_children() %>%
.[[1]]
xml_remove(x1)
rm(x1)
gc()
#> used (Mb) gc trigger (Mb) limit (Mb) max used (Mb)
#> Ncells 598977 32.0 1355532 72.4 NA 1320341 70.6
#> Vcells 1127775 8.7 8388608 64.0 32768 2040786 15.6
x2
#> {xml_node}
#> <baz>If you are not planning on referencing these nodes again this memory
is wasted. Calling xml_remove(free = TRUE) will remove the
nodes and free the memory used to store them.
Note In this case any node which previously
pointed to the node or its children will instead be pointing to free
memory and may cause R to crash. xml2 can’t figure this out for you, so
it’s your responsibility to remove any objects which are no longer
valid.
In particular xml_find_*() results are easy to overlook,
for example
x <- read_xml("<a><b /><b><b /></b></a>")
bees <- xml_find_all(x, "//b")
xml_remove(xml_child(x), free = TRUE)
# bees[[1]] is no longer valid!!!
rm(bees)
gc()
#> used (Mb) gc trigger (Mb) limit (Mb) max used (Mb)
#> Ncells 598883 32.0 1355532 72.4 NA 1320341 70.6
#> Vcells 1127612 8.7 8388608 64.0 32768 2040786 15.6We want to construct a document with the following namespace layout. (From https://stackoverflow.com/questions/32939229/creating-xml-in-r-with-namespaces/32941524#32941524).
<?xml version = "1.0" encoding="UTF-8"?>
<sld xmlns="http://www.opengis.net/sld"
xmlns:ogc="http://www.opengis.net/ogc"
xmlns:se="http://www.opengis.net/se"
version="1.1.0" >
<layer>
<se:Name>My Layer</se:Name>
</layer>
</sld>d <- xml_new_root("sld",
"xmlns" = "http://www.opengis.net/sld",
"xmlns:ogc" = "http://www.opengis.net/ogc",
"xmlns:se" = "http://www.opengis.net//se",
version = "1.1.0"
) %>%
xml_add_child("layer") %>%
xml_add_child("se:Name", "My Layer") %>%
xml_root()
d
#> {xml_document}
#> <sld version="1.1.0" xmlns="http://www.opengis.net/sld" xmlns:ogc="http://www.opengis.net/ogc" xmlns:se="http://www.opengis.net//se">
#> [1] <layer>\n <se:Name>My Layer</se:Name>\n</layer>