How to paste formula into model.matrix function in R?

Question

By way of simplified example, say you have the following data:

n <- 10
df <- data.frame(x1 = rnorm(n, 3, 1), x2 = rnorm(n, 0, 1))

And you wish to create a model matrix of the following form:

model.matrix(~ df$x1 + df$x2)

or more preferably:

model.matrix(~ x1 + x2, data = df)

but instead by pasting the formula into model.matrix . I have experimented with the following but encounter errors with all of them:

form1 <- "df$x1 + df$x2"
model.matrix(~ as.formula(form1))
model.matrix(~ eval(parse(text = form1)))
model.matrix(~ paste(form1))
model.matrix(~ form1)

I've also tried the same with the more preferable structure:

form2 <- "x1 + x2, data = df"

Is there a direct solution to this problem? Or is the model.matrix function not conducive to this approach?

Answer 1

Do you mean something like this?

expr <- "~ x1 + x2"
model.matrix(as.formula(expr), df)

You need to give df as the data argument outside of as.formula , as the data argument defines the environment within which to evaluate the formula.

If you don't want to specify the data argument you can do

model.matrix(as.formula("~ df$x1 + df$x2"))