R: Extract Something From a G1SST Object

[[,g1sst-method {oce} | R Documentation

## Extract Something From a G1SST Object

### Description

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.

### Usage

    ## S4 method for signature 'g1sst'
    x[[i, j, ...]]

### Arguments

`x` | a g1sst 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 `[[`. The `data` and `metadata` items hold the names of entries in the object's data and metadata slots, respectively. The `dataDerived` and `metadataDerived` items are each NULL, because no derived values are defined by g1sst 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.

1. 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.

2. 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.

3. If `i` is a string ending in `"Flag"`, then the corresponding data-quality flag is returned (or `NULL` if there is no such flag).

4. 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.

5. 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.

6. If none of the above-listed conditions holds, then `NULL` is returned.

### Author(s)

Dan Kelley

### 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`, `[[,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`, `[[,xbt-method`, `[[<-,adv-method`

Other things related to g1sst data: `[[<-,g1sst-method`, `g1sst-class`, `read.g1sst()`

[Package oce version 1.7-2 Index]