get a data.frame of attribute values of a given node
Details
level should be a character vector giving the path to the specified node
group. For instance, otus, characters, and trees are top-level blocks (e.g.
child nodes of the root nexml block), and can be specified directly. To get metadata
for all "char" elements from all characters blocks, you must specify that char nodes
are child nodes to character nodes: e.g. get_level(nex, "characters/char"),
or similarly for states: get_level(nex, characters/states).
The return object is a data frame whose columns are the attribute names of the elements
specified. The column names match the attribute names except for "id" attribute, for which the column
is renamed using the node itself. (Thus <otus id="os2"> would be rendered in a data.frame with column
called "otus" instead of "id"). Additional columns are
added for each parent element in the path; e.g. get_level(nex, "otus/otu") would include a column
named "otus" with the id of each otus block. Even though the method always returns the data frame
for all matching nodes in all blocks, these ids let you see which otu values came from which
otus block. This is identical to the function call get_taxa().
Similarly, get_level(nex, "otus/otu/meta") would return additional columns 'otus' and
also a column, 'otu', with the otu parent ids of each metadata block. (This is identical to a
function call to get_metadata). This makes it easier to join data.frames as well, see examples