The [[ method works for all oce objects, i.e.
objects inheriting from oce-class. The purpose,
as with the related replacement method, [[<-,
is to insulate users from the internal details of oce
objects, by looking for items within the various storage
slots of the object. Items not actually stored can also be
extracted, including derived data, units of measurement,
and data-quality flags.
The method uses a two-step process to try to find the
requested information. First, a class-specific function
is used to try to access the requested information (see
“Details of the specialized ... method”).
Second, if no match is found, a general function is used
(see ‘Details of the general method’). If neither
method can locates the requested item, NULL is returned.
# S4 method for section
[[(x, i, j, ...)A section object, i.e. one inheriting from section-class.
Character string indicating the name of item to extract.
Optional additional information on the i item.
Optional additional information (ignored).
There are several possibilities, depending on the nature of i.
If i is the string "station", then the method
will return a list of
ctd-class objects holding the station data.
If j is also given, it specifies a station (or set of stations) to be returned.
if j contains just a single value, then that station is returned, but otherwise
a list is returned. If j is an integer, then the stations are specified by index,
but if it is character, then stations are specified by the names stored within
their metadata. (Missing stations yield NULL in the return value.)
If i is "station ID", then the IDs of the stations in the
section are returned.
If i is "dynamic height", then an estimate of dynamic
height is returned (as calculated with swDynamicHeight(x)).
If i is "distance", then the distance along the section is
returned, using geodDist.
If i is "depth", then a vector containing the depths
of the stations is returned.
If i is "z", then a vector containing the z
coordinates is returned.
If i is "theta" or "potential temperature", then
the potential temperatures of all the stations are returned in one
vector. Similarly, "spice" returns the property known
as spice, using swSpice.
If i is a string ending with "Flag", then the characters
prior to that ending are taken to be the name of a variable contained
within the stations in the section. If this flag is available in
the first station of the section, then the flag values are looked
up for every station.
If none of the conditions listed above holds, the general method is used (see ‘Details of the general method’).
If the specialized method produces no matches, the following generalized
method is applied. As with the specialized method, the procedure hinges
first on the value of i.
First, a check is made as to whether i names one of the standard
oce slots, and returns the slot contents if so. Thus,
x[["metadata"]] will retrieve the metadata slot,
while x[["data"]] and x[["processingLog"]] return
those slots.
Next, 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"]], then just the expression is returned,
and if it ends in " scale", then just the scale is returned.
Next, 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 none of the preceding conditions are met, a check is done
to see if the metadata slot contains an item with
the provided name, and that is returned, if so. A direct match
is required for this condition.
Finally, the data slot is checked to see if it contains
an item with the name indicated by i. In this case, a partial
match will work; this is accomplished by using pmatch.
If none of the above-listed conditions holds, then NULL is returned.
Other things related to section data: [[<-,section-method,
as.section,
handleFlags,section-method,
plot,section-method,
read.section, section-class,
sectionAddStation,
sectionGrid, sectionSmooth,
sectionSort, section,
subset,section-method,
summary,section-method
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,
[[,lisst-method,
[[,lobo-method,
[[,met-method, [[,odf-method,
[[,rsk-method,
[[,sealevel-method,
[[,tidem-method,
[[,topo-method,
[[,windrose-method,
[[<-,adv-method
# NOT RUN {
data(section)
length(section[["latitude"]])
length(section[["latitude", "byStation"]])
data(section)
length(section[["latitude"]])
length(section[["latitude", "byStation"]])
# }
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