Thank you for your answers, I have another question: the behaviour of setdiff(indicesFalse, indicesNA) does not seem predictable to me.
indices
[1] 1 2 3 4 5 6
compareVector
[1] NA TRUE TRUE TRUE FALSE NA
indicesNA = indices[is.na(compareVector)] indicesNA
[1] 1 6
indicesFalse = indices[compareVector == FALSE] indicesFalse
[1] NA 5 NA
setdiff(indicesNA, indicesFalse) ######################## OK
[1] 1 6
setdiff(indicesFalse, indicesNA) ######################## I would expect here 'NA 5 NA'
[1] NA 5
Svetlana Eden Biostatistician II School of Medicine
Department of Biostatistics Vanderbilt University