The [[ method works for all oce objects, i.e.
objects inheriting from oce-class. The purpose
is to insulate users from the internal details of oce
objects, by looking for items within the various storage
slots of the object. Items that are not actually stored in
the object can also be extracted, including derived data such
as potential temperature, the units of measurement for
the data, data-quality flags, etc.
# S4 method for cm
[[(x, i, j, ...)A cm object, i.e. one inheriting from cm-class.
Character string indicating the name of item to extract.
Optional additional information on the i item.
Optional additional information (ignored).
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. 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).
For example, x[["salinityFlag"]] returns a vector of salinity
flags if x is a ctd object.
If i is "sigmaTheta", then the value of
swSigmaTheta(x) is returned. Similarly,
"sigma0" returns swSigma0(x) and
"spice" returns swSpice(x). Note that
these relate to seawater properties and only make sense for
certain object types.
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,
without the issuance of a warning or error message. (This silent operation
is employed so that [[ will behave like the normal R version.)
A two-step process is used to try to find the requested information. First, a class-specific function tries to find it, but if that fails, then a general function is used (see next section).
Other functions that extract parts of oce objects: [[,adp-method,
[[,adv-method,
[[,amsr-method,
[[,argo-method,
[[,bremen-method,
[[,coastline-method,
[[,ctd-method,
[[,echosounder-method,
[[,g1sst-method,
[[,gps-method,
[[,ladp-method,
[[,landsat-method,
[[,lisst-method,
[[,lobo-method,
[[,met-method, [[,oce-method,
[[,odf-method, [[,rsk-method,
[[,sealevel-method,
[[,section-method,
[[,tidem-method,
[[,topo-method,
[[,windrose-method,
[[<-,adv-method
Other things related to cm data: [[<-,cm-method,
as.cm, cm-class,
cm, plot,cm-method,
read.cm, rotateAboutZ,
subset,cm-method,
summary,cm-method