Making compatible (equal) dimensions for two vectors in R

Question

I have a vector called classes that is the output of an analysis that used listwise deletion. As a result, the cases included in classes is a subset of the entire dataset -- some cases were dropped because of incomplete data.

Selection is a dummy variable that occurs with every case in my dataset. A shortened example of my data is below. There is also a unique case ID for every observation.

classes <- c(1,2,1,1,1,2,3,3,3,1,1,1,3,3,2,2,2)
selection <- c(1,0,0,0,1,1,1,1,0,0,0,0,0,1,1,1,1,0,0,0,1,1,1,0,1,0)
case <-seq(1,26,1)

I would like to create a new version of selection (say, selection2) so that it only includes cases that are in classes. Basically, I would like both variables to be the same length for comparison purposes, where the cases that are NOT included in classes are also not included in selection2.

I thought this would be an easy fix, but I've spend a lot of time getting nowhere, so I thought I'd ask. Thanks in advance!

Answer 1

If they are to be the same length, then the reduced version must have NA's:

> selection2 <- selection
> is.na(selection2) <- !selection2 %in% classes
> selection2
 [1]  1 NA NA NA  1  1  1  1 NA NA NA NA NA  1  1  1  1 NA NA NA  1  1  1 NA  1 NA