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.
Arguments
- x
an xbt object.
- i
character value indicating the name of an item to extract.
- j
optional additional information on the
i
item.- ...
ignored.
Details
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.
Details of the Specialized Method
If
i
is"?"
, 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[[
. Thedata
andmetadata
items hold the names of entries in the object's data and metadata slots, respectively. ThedataDerived
andmetadataDerived
items are each NULL, because no derived values are defined bycm
objects.
Details of the General Method
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 standardoce
slots. If so,[[
returns the slot contents of that slot. Thus,x[["metadata"]]
will retrieve themetadata
slot, whilex[["data"]]
andx[["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 (orNULL
if there is no such unit). This list consists of an item namedunit
, which is anexpression()
, and an item namedscale
, 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 (orNULL
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 ofswSigmaTheta()
is called withx
as the sole argument, and the results are returned. Similarly,swSigma0()
is used ifi="sigma0"
, andswSpice()
is used ifi="spice"
. Of course, these actions only make sense for objects that contain the relevant items within theirdata
slot.After these possibilities are eliminated, the action depends on whether
j
has been provided. Ifj
is not provided, or is the string""
, theni
is sought in themetadata
slot, and then in thedata
slot, returning whichever is found first. In other words, ifj
is not provided, themetadata
slot takes preference over thedata
slot. However, ifj
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.
See also
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
,
[[,oce-method
,
[[,odf-method
,
[[,rsk-method
,
[[,sealevel-method
,
[[,section-method
,
[[,tidem-method
,
[[,topo-method
,
[[,windrose-method
,
[[<-,adv-method
Other things related to xbt data:
[[<-,xbt-method
,
as.xbt()
,
plot,xbt-method
,
read.xbt()
,
read.xbt.noaa1()
,
subset,xbt-method
,
summary,xbt-method
,
xbt
,
xbt-class
,
xbt.edf