Author: Jim Hester
Date: 2020-03-31

# Node Modification

#### Jim Hester

#### 2020-03-31

# Modifying Existing XML

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

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}

## Attribute and Namespace Definition Modification

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">

## Name Modification

Node names are modified with `xml_name()`.

    x <- read_xml("<a><b/></a>")
    x
    #> {xml_document}
    #> <a>
    #> [1] <b/>
    xml_name(x)
    #> [1] "a"
    xml_name(x) <- "c"
    x
    #> {xml_document}
    #> <c>
    #> [1] <b/>

# Node modification

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.

## Replacing existing nodes

    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>

## Add a sibling

    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>

## Add a 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>

## Removing nodes

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  572101 30.6    1275841 68.2         NA  1272283 68.0
    #> Vcells 1100538  8.4    8388608 64.0      32768  2033871 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  572148 30.6    1275841 68.2         NA  1272283 68.0
    #> Vcells 1100589  8.4    8388608 64.0      32768  2033871 15.6

## Namespaces

We want to construct a document with the following namespace layout. (From http://stackoverflow.com/questions/32939229/creating-xml-in-r-with-namespaces/32941524#32941524).

    <?xml version = "1.0" encoding="UTF-8"?>
    <sld xmlns="http://www.o.net/sld"
         xmlns:ogc="http://www.o.net/ogc"
         xmlns:se="http://www.o.net/se"
         version="1.1.0" >
    <layer>
    <se:Name>My Layer</se:Name>
    </layer>
    </sld>

    d <- xml_new_root("sld",
        xmlns = "http://www.o.net/sld",
        "xmlns:ogc" = "http://www.o.net/ogc",
        "xmlns:se" = "http://www.o.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.o.net/sld" xmlns:ogc="http://www.o.net/ogc" xmlns:se="http://www.o.net/se">
    #> [1] <layer>\n  <se:Name>My Layer</se:Name>\n</layer>