R: Retrieve an attribute.

xml_attr {xml2} | R Documentation

## Retrieve an attribute.

### Description

`xml_attrs()` retrieves all attributes values as a named character vector, `xml_attrs() <-` or `xml_set_attrs()` sets all attribute values. `xml_attr()` retrieves the value of single attribute and `xml_attr() <-` or `xml_set_attr()` modifies its value. If the attribute doesn't exist, it will return `default`, which defaults to `NA`. `xml_has_attr()` tests if an attribute is present.

### Usage

    xml_attr(x, attr, ns = character(), default = NA_character_)

    xml_has_attr(x, attr, ns = character())

    xml_attrs(x, ns = character())

    xml_attr(x, attr, ns = character()) <- value

    xml_set_attr(x, attr, value, ns = character())

    xml_attrs(x, ns = character()) <- value

    xml_set_attrs(x, value, ns = character())

### Arguments

`x` | A document, node, or node set.
`attr` | Name of attribute to extract.
`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()`.
`default` | Default value to use when attribute is not present.
`value` | character vector of new value.

### Value

`xml_attr()` returns a character vector. `NA` is used to represent of attributes that aren't defined.

`xml_has_attr()` returns a logical vector.

`xml_attrs()` returns a named character vector if `x` x is single node, or a list of character vectors if given a nodeset

### Examples

    x <- read_xml("<root id='1'><child id ='a' /><child id='b' d='b'/></root>")
    xml_attr(x, "id")
    xml_attr(x, "apple")
    xml_attrs(x)

    kids <- xml_children(x)
    kids
    xml_attr(kids, "id")
    xml_has_attr(kids, "id")
    xml_attrs(kids)

    # Missing attributes give missing values
    xml_attr(xml_children(x), "d")
    xml_has_attr(xml_children(x), "d")

    # If the document has a namespace, use the ns argument and
    # qualified attribute names
    x <- read_xml('
     <root xmlns:b="http://bar.com" xmlns:f="http://foo.com">
       <doc b:id="b" f:id="f" id="" />
     </root>
    ')
    doc <- xml_children(x)[[1]]
    ns <- xml_ns(x)

    xml_attrs(doc)
    xml_attrs(doc, ns)

    # If you don't supply a ns spec, you get the first matching attribute
    xml_attr(doc, "id")
    xml_attr(doc, "b:id", ns)
    xml_attr(doc, "id", ns)

    # Can set a single attribute with `xml_attr() <-` or `xml_set_attr()`
    xml_attr(doc, "id") <- "one"
    xml_set_attr(doc, "id", "two")

    # Or set multiple attributes with `xml_attrs()` or `xml_set_attrs()`
    xml_attrs(doc) <- c("b:id" = "one", "f:id" = "two", "id" = "three")
    xml_set_attrs(doc, c("b:id" = "one", "f:id" = "two", "id" = "three"))

[Package xml2 version 1.3.0 Index]