[R] Formulae with factors that have missing values

[Rachel Merriman]

Hi All,

I have a formula which has a factor with NAs in it.  I wish to keep
these in the model matrix, but the NA information is currently lost (the
rows are kept but the NA gets converted to 0).  Any ideas as to how
I can keep NAs in?

e.g.

junk <-
factor(c("hi",NA,"low","low","hi","low","hi","hi","low",NA,"hi","low","hi","hi","low","hi"))

y <- c(1,2,1,2,2,2,1,2,1,1,2,2,1,1,1,2)

na.keep <- function(X){X}

myfn <- function (formula,data=sys.parent()){
    mf <- match.call()
    mf[[1]] <- as.name("model.frame")
    if(is.null(mf$na.action)) mf$na.action <- as.name("na.keep")
    mf <- eval(mf, sys.frame(sys.parent()))
    Y <- model.extract(mf,response)
    Terms <- attr(mf,"terms")
    X <- model.matrix(Terms,mf)
    X
}

myfn(y~junk)

Cheers,
Rachel

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