[[ method lets users extract information from
objects, without having to know the details of the internal storage. For
[[ can also return quantities that are computed
from the object's contents.
# S4 method for xbt [[(x, i, j, ...)
an xbt object.
character value indicating the name of an item to extract.
optional additional information on the
A two-step process is used to try to find the requested information. First, a
class-specific function is used (see “Details of the Specialized
Method”). If this yields nothing, then a general method is used (see
“Details of the General Method”). If both methods fail, then
Some understanding of the subclass is required to know what can be retrieved
[[. When dealing with an unfamiliar subclass, it can be useful to
x[["?"]] to get a listing of the retrievable items. See
“Details of the Specialized Method” for more information.
"?", then the return value is a list
containing four items, each of which is a character vector
holding the names of things that can be accessed with
metadata items hold the names of
entries in the object's data and metadata
slots, respectively. The
metadataDerived items are each NULL, because
no derived values are defined by
Note: the text of this section is identical for all
oce subclasses, and so
some of what you read here may not be relevant to the class being described
in this help page.
If the specialized method produces no matches, the following generalized
method is applied. As with the specialized method, the procedure hinges first
on the values of
i and, optionally,
j. The work proceeds in steps, by
testing a sequence of possible conditions in sequence.
A check is made as to whether
i names one of the standard
[[ returns the slot contents of that slot. Thus,
will retrieve the
metadata slot, while
x[["processingLog"]] return those slots.
i is a string ending in the
"Unit", then the characters preceding
that string are taken to be the name of an item in the data object, and a
list containing the unit is returned (or
NULL if there is no such unit).
This list consists of an item named
unit, which is an
an item named
scale, which is a string describing the measurement scale.
If the string ends in
" unit", e.g.
x[["temperature unit"]] (note the
space), then just the expression is returned, and if it ends in
then just the scale is returned.
i is a string ending in
"Flag", then the corresponding data-quality
flag is returned (or
NULL if there is no such flag).
If the object holds hydrographic information (pressure, salinity,
temperature, longitude and latitude) then another set of possibilities
"sigmaTheta", then the value of
x as the sole argument, and the results are returned.
swSigma0() is used if
swSpice() is used if
i="spice". Of course, these actions only make sense for objects that
contain the relevant items within their
After these possibilities are eliminated, the action depends on whether
j has been provided. If
j is not provided, or is the string
i is sought in the
metadata slot, and then in the
data slot, returning
whichever is found first. In other words, if
j is not provided, the
metadata slot takes preference over the
data slot. However, if
provided, then it must be either the string
"data", and it
directs where to look.
If none of the above-listed conditions holds, then
NULL is returned.
Other functions that extract parts of oce objects: