Generally, the [[ method lets users extract information from oce
objects, without having to know the details of the internal storage. For
many oce sub-classes, [[ can also return quantities that are computed
from the object's contents.
# S4 method for oce
[[(x, i, j, ...)an oce object.
character value indicating the name of an item to extract.
optional additional information on the i item.
ignored.
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 oce slots.
If so, [[ returns the slot contents of that slot. Thus, x[["metadata"]]
will retrieve the metadata slot, while x[["data"]] and
x[["processingLog"]] return those slots.
If 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 expression(), and
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 " scale",
then just the scale is returned.
If 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
arises. If i is "sigmaTheta", then the value of swSigmaTheta() is
called with x as the sole argument, and the results are returned.
Similarly, swSigma0() is used if i="sigma0", and swSpice() is used if
i="spice". Of course, these actions only make sense for objects that
contain the relevant items within their data slot.
After these possibilities are eliminated, the action depends on whether
j has been provided. If j is not provided, or is the string "", then
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 j is
provided, then it must be either the string "metadata" or "data", and it
directs where to look.
If none of the above-listed conditions holds, then NULL is returned.
Dan Kelley
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 [[
returns NULL.
Some understanding of the subclass is required to know what can be retrieved
with [[. When dealing with an unfamiliar subclass, it can be useful to
first use x[["?"]] to get a listing of the retrievable items. See
“Details of the Specialized Method” for more information.
Many oce object classes have specialized versions
of [[ that handle the details in specialized way.
Other functions that extract parts of oce objects:
[[,adp-method,
[[,adv-method,
[[,amsr-method,
[[,argo-method,
[[,bremen-method,
[[,cm-method,
[[,coastline-method,
[[,ctd-method,
[[,echosounder-method,
[[,g1sst-method,
[[,gps-method,
[[,ladp-method,
[[,landsat-method,
[[,lisst-method,
[[,lobo-method,
[[,met-method,
[[,odf-method,
[[,rsk-method,
[[,sealevel-method,
[[,section-method,
[[,tidem-method,
[[,topo-method,
[[,windrose-method,
[[,xbt-method,
[[<-,adv-method