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

## Extract Something From a Section Object

### Description

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.

### Usage

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

### Arguments

`x` | A `section` object, i.e. one inheriting from `section-class`.
`i` | Character string indicating the name of item to extract.
`j` | Optional additional information on the `i` item.
`...` | Optional additional information (ignored).

### Details of the specialized section method

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’).

### 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.

### Author(s)

Dan Kelley

### See Also

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`

### Examples

    data(section)
    length(section[["latitude"]])
    length(section[["latitude", "byStation"]])

    data(section)
    length(section[["latitude"]])
    length(section[["latitude", "byStation"]])

[Package oce version 0.9-22 Index]