suggested addition to model.matrix

-----Original Message-----
From: R-devel [mailto:r-devel-bounces at r-project.org] On Behalf Of Spencer
Graves
Sent: October 3, 2016 7:59 PM
To: r-devel at r-project.org
Subject: [Rd] suggested addition to model.matrix

Hello, All:


       What's the simplest way to convert a data.frame into a model.matrix?


       One way is given by the following example, modified from the examples in
help(model.matrix):


dd <- data.frame(a = gl(3,4), b = gl(4,1,12))
ab <- model.matrix(~ a + b, dd)
ab0 <- model.matrix(~., dd)
all.equal(ab, ab0)


       What do you think about replacing "model.matrix(~ a + b, dd)" in
the current help(model.matrix) with this 3-line expansion?


       I suggest this, because I spent a few hours today trying to
convert a data.frame into a model.matrix before finding this.


       Also, what do you think about adding something like the following
to the stats package:


model.matrix.data.frame <- function(object, ...){
     model.matrix(~., object, ...)
}


       And then extend the above example as follows:

ab. <- model.matrix(dd)
all.equal(ab, ab.)


       Thanks,
       Spencer Graves

Dear Spencer,

I don't think that the problem of "converting a data frame into a model
matrix" is well-defined, because there isn't a unique mapping from one to
the other.

In your example, you build  the model matrix for the additive formula ~ a
+ b from the data frame matrix containing a and b, using "treatment"
contrasts, but there are other possible formulas (e.g., ~ a*b) and
contrasts [e.g., model.matrix(~ a + b, dd, contrasts=list(a=contr.sum,
b=contr.helmert)].

So I think that the current approach is sensible -- to require both a data
frame and a formula.

Best,
 John

-----Original Message-----
From: R-devel [mailto:r-devel-bounces at r-project.org] On Behalf Of

Spencer

Graves
Sent: October 3, 2016 7:59 PM
To: r-devel at r-project.org
Subject: [Rd] suggested addition to model.matrix

Hello, All:


       What's the simplest way to convert a data.frame into a

model.matrix?

       One way is given by the following example, modified from the

examples in

help(model.matrix):


dd <- data.frame(a = gl(3,4), b = gl(4,1,12))
ab <- model.matrix(~ a + b, dd)
ab0 <- model.matrix(~., dd)
all.equal(ab, ab0)


       What do you think about replacing "model.matrix(~ a + b, dd)" in
the current help(model.matrix) with this 3-line expansion?


       I suggest this, because I spent a few hours today trying to
convert a data.frame into a model.matrix before finding this.


       Also, what do you think about adding something like the following
to the stats package:


model.matrix.data.frame <- function(object, ...){
     model.matrix(~., object, ...)
}


       And then extend the above example as follows:

ab. <- model.matrix(dd)
all.equal(ab, ab.)


       Thanks,
       Spencer Graves