[[ 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 argo [[(x, i, j, ...)
an argo 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.
Note that argo data may contain both unadjusted data and adjusted
data. By default, this extraction function refers to the former, but a
preference for the latter may be set with
documentation of which explains (fairly complex) details.
The results from
argo[[i,j]] depend on the
i and (if provided)
j. The details are as follows.
"?", 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 hold the names of things
that can be inferred from the object's contents, e.g.
"SA" is named in
dataDerived, indicating that
argo[["SA"]] is permitted (to compute Absolute Salinity).
j is an integer vector,
then an argo object is returned, as specified by
j. For example,
argo[["profile", 2:5]] is equivalent to
subset(argo, profile %in% 2:5).
Conservative Temperature is returned, as computed with
SA is computed as explained
in the next item,
t is in-situ temperature,
p is pressure.
"N2", then the square of buoyancy is returned,
as computed with
Absolute Salinity is returned, as computed with
then the associated function in the gsw package.
gsw::gsw_sigma0(), which returns
potential density anomaly referenced to 0 dbar,
according to the gsw equation of state.
a matrix of depths is returned.
"ID", then the
id element within
metadata slot is returned.
i is in the
data slot of
then it is returned, otherwise if it is in the
then that is returned, otherwise
NULL is returned.
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:
Other things related to argo data:
data(argo) # 1. show that dataset has 223 profiles, each with 56 levels dim(argo[['temperature']]) #>  56 223 # 2. show importance of focussing on data flagged 'good' fivenum(argo[["salinity"]],na.rm=TRUE) #>  0.000 34.890 34.926 35.042 47.899 fivenum(argo[["salinity"]][argo[["salinityFlag"]]==1],na.rm=TRUE) #>  32.850 34.890 34.926 35.042 35.842