model.matrix() with na.action=NULL?

Question

I have a formula and a data frame, and I want to extract the model.matrix(). However, I need the resulting matrix to include the NAs that were found in the orig

Answer 1

Joris's suggestion works, but a quicker and cleaner way to do this is via the global na.action setting. The 'Pass' option achieves our goal of preserving NA's from the original dataset.

Option 1: Pass

Resulting matrix will contain NA's in rows corresponding to the original dataset.

options(na.action='na.pass')
model.matrix(ff, dat) 

Option 2: Omit

Resulting matrix will skip rows containing NA's.

options(na.action='na.omit')
model.matrix(ff, dat) 

Option 3: Fail

An error will occur if the original data contains NA's.

options(na.action='na.fail')
model.matrix(ff, dat) 

Of course, always be careful when changing global options because they can alter behavior of other parts of your code. A cautious person might store the original setting with something like current.na.action <- options('na.action'), and then change it back after making the model.matrix.