R: Extract Something From an ladp Object

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

## Extract Something From an ladp Object

### Description

The `[[` method works for all `oce` objects, i.e. objects inheriting from `oce-class`. The purpose 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.

### Usage

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

### Arguments

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

### Details

A two-step process is used to try to find the requested information. First, a class-specific function tries to find it (see “Details of the specialized ... method”), but if that fails, then a general function is used (see ‘Details of the general method’). If both fail, `NULL` is returned.

### 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 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`, `[[,landsat-method`, `[[,lisst-method`, `[[,lobo-method`, `[[,met-method`, `[[,odf-method`, `[[,rsk-method`, `[[,sealevel-method`, `[[,section-method`, `[[,tidem-method`, `[[,topo-method`, `[[,windrose-method`, `[[<-,adv-method`

Other things related to `ladp` data: `[[<-,ladp-method`, `as.ladp`, `ladp-class`, `plot,ladp-method`, `summary,ladp-method`

### Examples

    data(ctd)
    head(ctd[["temperature"]])

[Package oce version 1.0-1 Index]