[BioC] help regarding tutorial Lab5 and Lab6

[dibakar ray]

Would any one explain what is the meaning of the following lines of code 
(this is part of the tutorial Computational and Statistical Aspects of 
Microarray Analysis, University of Milan, May 2003,
Lab5 and Lab6)
  --------- Problem no 1 ---------------
# DO NOT UNDERSTAND THE FOLLOWING LINES.

 > golubTrainSub <- golubTrain[sub, ]
 > golubTrainSub at exprs <- X
 > Y <- golubTrainSub$ALL.AML

#I UNDERSTAND THE FOLLOWING LINE
 > Y <- paste(golubTrain$ALL.AML, golubTrain$T.B.cell)

# DO NOT UNDERSTAND THE FOLLOWING ONE.
 > Y <- sub("NA", "", Y)
---------- Problem no 2 ----------------------
 From Lab6 Code, the following is OK
================================
 > data(golubTrain)
 > data(golubMerge)
 > data(golubTest)
 > gTrn <- GolubTrans(golubTrain)
 > gTest <- GolubTrans(golubTest)
 > gMerge <- GolubTrans(golubMerge)
 > mmfun <- mmfilt()
 > ffun <- filterfun(mmfun)
 > sub <- genefilter(gTrn, ffun)

But why we need to set the following element False?

 > sub[c(2401, 3398, 4168)] <- FALSE
===========================??????? ===================
Thanks in advance
Dibakar Ray