[R] extract data from lm object and then use again?

[Marc Schwartz (via MN)]

On Fri, 2006-09-22 at 10:45 -0400, Mike Wolfgang wrote:
> Hi list,
> 
> I want to write a general function so that it would take an lm object,
> extract its data element, then use the data at another R function (eg, glm).
> I searched R-help list, and found this would do the trick of the first part:
> a.lm$call$data
> this would return a name object but could not be recognized as a
> data.frameby glm. I also tried
> call(as.character(a.lm$call$data))
> or
> eval(call(as.character(a.lm$call$data)))
> neither works.
> 
> By eval(call(...)), it acts as evaluating of a function, but what I want is
> just a data frame object which could be inserted into glm function. Anyone
> could help? Thanks,
> 
> Mike

If the 'data' argument in lm() is used, then this approach could work:

> Iris2 <- eval(lm(Sepal.Length ~ Species, data = iris)$call$data)

> str(Iris2)
`data.frame':   150 obs. of  5 variables:
 $ Sepal.Length: num  5.1 4.9 4.7 4.6 5 5.4 4.6 5 4.4 4.9 ...
 $ Sepal.Width : num  3.5 3 3.2 3.1 3.6 3.9 3.4 3.4 2.9 3.1 ...
 $ Petal.Length: num  1.4 1.4 1.3 1.5 1.4 1.7 1.4 1.5 1.4 1.5 ...
 $ Petal.Width : num  0.2 0.2 0.2 0.2 0.2 0.4 0.3 0.2 0.2 0.1 ...
 $ Species     : Factor w/ 3 levels "setosa","versicolor",..: 1 1 1 1 1
1 1 1 1 1 ...


However, note that you do not get the actual data used within the lm()
function (the model frame) but the entire source data frame.

What you likely want instead is the model frame containing the columns
actually used in the model formula:

> Iris3 <- lm(Sepal.Length ~ Species, data = iris)$model

> str(Iris3)
`data.frame':   150 obs. of  2 variables:
 $ Sepal.Length: num  5.1 4.9 4.7 4.6 5 5.4 4.6 5 4.4 4.9 ...
 $ Species     : Factor w/ 3 levels "setosa","versicolor",..: 1 1 1 1 1
1 1 1 1 1 ...
 - attr(*, "terms")=Classes 'terms', 'formula' length 3 Sepal.Length ~
Species
  .. ..- attr(*, "variables")= language list(Sepal.Length, Species)
  .. ..- attr(*, "factors")= int [1:2, 1] 0 1
  .. .. ..- attr(*, "dimnames")=List of 2
  .. .. .. ..$ : chr [1:2] "Sepal.Length" "Species"
  .. .. .. ..$ : chr "Species"
  .. ..- attr(*, "term.labels")= chr "Species"
  .. ..- attr(*, "order")= int 1
  .. ..- attr(*, "intercept")= int 1
  .. ..- attr(*, "response")= int 1
  .. ..- attr(*, ".Environment")=length 15 <environment>
  .. ..- attr(*, "predvars")= language list(Sepal.Length, Species)
  .. ..- attr(*, "dataClasses")= Named chr [1:2] "numeric" "factor"
  .. .. ..- attr(*, "names")= chr [1:2] "Sepal.Length" "Species"


HTH,

Marc Schwartz