R: Extracting the Model Frame from a Formula or Fit

model.frame {stats} | R Documentation

## Extracting the Model Frame from a Formula or Fit

### Description

`model.frame` (a generic function) and its methods return a `data.frame` with the variables needed to use `formula` and any `...` arguments.

### Usage

    model.frame(formula, ...)

    ## Default S3 method:
    model.frame(formula, data = NULL,
               subset = NULL, na.action = na.fail,
               drop.unused.levels = FALSE, xlev = NULL, ...)

    ## S3 method for class 'aovlist'
    model.frame(formula, data = NULL, ...)

    ## S3 method for class 'glm'
    model.frame(formula, ...)

    ## S3 method for class 'lm'
    model.frame(formula, ...)

    get_all_vars(formula, data, ...)

### Arguments

`formula` | a model `formula` or `terms` object or an R object.
`data` | a data.frame, list or environment (or object coercible by `as.data.frame` to a data.frame), containing the variables in `formula`. Neither a matrix nor an array will be accepted.
`subset` | a specification of the rows to be used: defaults to all rows. This can be any valid indexing vector (see `[.data.frame`) for the rows of `data` or if that is not supplied, a data frame made up of the variables used in `formula`.
`na.action` | how `NA`s are treated. The default is first, any `na.action` attribute of `data`, second a `na.action` setting of `options`, and third `na.fail` if that is unset. The ‘factory-fresh’ default is `na.omit`. Another possible value is `NULL`.
`drop.unused.levels` | should factors have unused levels dropped? Defaults to `FALSE`.
`xlev` | a named list of character vectors giving the full set of levels to be assumed for each factor.
`...` | further arguments such as `data`, `na.action`, `subset`. Any additional arguments such as `offset` and `weights` which reach the default method are used to create further columns in the model frame, with parenthesised names such as `"(offset)"`.

### Details

Exactly what happens depends on the class and attributes of the object `formula`. If this is an object of fitted-model class such as `"lm"`, the method will either return the saved model frame used when fitting the model (if any, often selected by argument `model = TRUE`) or pass the call used when fitting on to the default method. The default method itself can cope with rather standard model objects such as those of class `"lqs"` from package MASS if no other arguments are supplied.

The rest of this section applies only to the default method.

If either `formula` or `data` is already a model frame (a data frame with a `"terms"` attribute) and the other is missing, the model frame is returned. Unless `formula` is a terms object, `as.formula` and then `terms` is called on it. (If you wish to use the `keep.order` argument of `terms.formula`, pass a terms object rather than a formula.)

Row names for the model frame are taken from the `data` argument if present, then from the names of the response in the formula (or rownames if it is a matrix), if there is one.

All the variables in `formula`, `subset` and in `...` are looked for first in `data` and then in the environment of `formula` (see the help for `formula()` for further details) and collected into a data frame. Then the `subset` expression is evaluated, and it is used as a row index to the data frame. Then the `na.action` function is applied to the data frame (and may well add attributes). The levels of any factors in the data frame are adjusted according to the `drop.unused.levels` and `xlev` arguments: if `xlev` specifies a factor and a character variable is found, it is converted to a factor (as from R 2.10.0).

Unless `na.action = NULL`, time-series attributes will be removed from the variables found (since they will be wrong if `NA`s are removed).

Note that all the variables in the formula are included in the data frame, even those preceded by `-`.

Only variables whose type is raw, logical, integer, real, complex or character can be included in a model frame: this includes classed variables such as factors (whose underlying type is integer), but excludes lists.

`get_all_vars` returns a `data.frame` containing the variables used in `formula` plus those specified `...`. Unlike `model.frame.default`, it returns the input variables and not those resulting from function calls in `formula`.

### Value

A `data.frame` containing the variables used in `formula` plus those specified in `...`. It will have additional attributes, including `"terms"` for an object of class `"terms"` derived from `formula`, and possibly `"na.action"` giving information on the handling of `NA`s (which will not be present if no special handling was done, e.g. by `na.pass`).

### References

Chambers, J. M. (1992) Data for models. Chapter 3 of Statistical Models in S eds J. M. Chambers and T. J. Hastie, Wadsworth & Brooks/Cole.

### See Also

`model.matrix` for the ‘design matrix’, `formula` for formulas and `expand.model.frame` for model.frame manipulation.

### Examples

    data.class(model.frame(dist ~ speed, data = cars))

[Package stats version 3.1.2 Index]