R: Look up an element via the XML catalog mechanism

catalogResolve {XML} | R Documentation

## Look up an element via the XML catalog mechanism

### Description

XML parsers use a catalog to map generic system and public addresses to actual local files or potentially different remote files. We can use a catalog to map a reference such as `http://www.omegahat.net/XSL/` to a particular directory on our local machine and then not have to modify any of the documents if we move the local files to another directory, e.g. install a new version in an alternate directory.

This function provides a mechanism to query the catalog to resolve a URI, PUBLIC or SYSTEM identifier.

This is now vectorized, so accepts a character vector of URIs and recycles `type` to have the same length.

If an entry is not resolved via the catalog system, a `NA` is returned for that element. To leave the value unaltered in this case, use `asIs = TRUE` .

### Usage

    catalogResolve(id, type = "uri", asIs = FALSE, debug = FALSE)

### Arguments

`id` | the name of the (generic) element to be resolved
`type` | a string, specifying whether the lookup is for a uri, system or public element
`asIs` | a logical. If `TRUE` any element of `id` which is not resolved by the catalog system will be left as given in the call. If `FALSE`, such unresolved elements are identified by `NA`.
`debug` | logical value indicating whether to turn on debugging output written to the console (`TRUE`) or not (`FALSE`).

### Value

A character vector. If the element was resolved, the single element is the resolved value. Otherwise, the character vector will contain no elements.

### Author(s)

Duncan Temple Lang

### References

http://www.xmlsoft.org http://www.sagehill.net/docbookxsl/Catalogs.html provides a short, succinct tutorial on catalogs.

### See Also

`xmlTreeParse`

### Examples

    if(!exists("Sys.setenv")) Sys.setenv = Sys.putenv

    Sys.setenv("XML_CATALOG_FILES" = system.file("exampleData", "catalog.xml", package = "XML"))

    catalogResolve("-//OASIS//DTD DocBook XML V4.4//EN", "public")

    catalogResolve("http://www.omegahat.net/XSL/foo.xsl")

    catalogResolve("http://www.omegahat.net/XSL/article.xsl", "uri")
    catalogResolve("http://www.omegahat.net/XSL/math.xsl", "uri")

      # This one does not resolve anything, returning an empty value.
    catalogResolve("http://www.oasis-open.org/docbook/xml/4.1.2/foo.xsl", "uri")

       # Vectorized and returns NA for the first and /tmp/html.xsl
       # for the second.

     catalogAdd("http://made.up.domain", "/tmp")
     catalogResolve(c("ddas", "http://made.up.domain/html.xsl"), asIs = TRUE)

[Package XML version 3.98-1.19 Index]