Get the value of an attribute in an XML node

This is a convenience function that retrieves the value of a named attribute in an XML node, taking care of checking for its existence. It also allows the caller to provide a default value to use as the return value if the attribute is not present.

Usage

xmlGetAttr(node, name, default = NULL, converter = NULL,
            namespaceDefinition = character(),
             addNamespace = length(grep(":", name)) > 0)

Arguments

node: the XML node
name: the name of the attribute
default: a value to use as the default return if the attribute is not present in the XML node.
converter: an optional function which if supplied is invoked with the attribute value and the value returned. This can be used to convert the string to an arbitrary value which is useful if it is, for example, a number. This is only called if the attribute exists within the node. In other words, it is not applied to the default value.
namespaceDefinition: a named character vector giving name space prefixes and URIs to use when resolving for the the attribute with a namespace. The values are used to compare the name space prefix used in the name given by the user to the name space definition in the node to ensure they match. This is important as we might ask for an attribute named r:width assuming that the prefix r corresponded to the URI http://www.r-project.org. However, there may be a name space prefix r defined on the node that points to a different URI and so this would be an erroneous match.
addNamespace: a logical value that indicates whether we should put the namespace prefix on the resulting name. This is passed on to xmlAttrs and so controls whether the resulting attribute names have the prefix attached. So one specifies TRUE for this argument if the attribute identifier has a namespace prefix.

Details

This just checks that the attribute list is non-NULL and that there is an element with the specified name.

Value

If the attribute is present, the return value is a string which is the value of the attribute. Otherwise, the value of default is returned.

References

https://www.w3.org/XML/, http://www.jclark.com/xml/, https://www.omegahat.net

Author

Duncan Temple Lang

Examples

 node <- xmlNode("foo", attrs=c(a="1", b="my name"))

 xmlGetAttr(node, "a")
#> [1] "1"
 xmlGetAttr(node, "doesn't exist", "My own default value")
#> [1] "My own default value"

 xmlGetAttr(node, "b", "Just in case")
#> [1] "my name"