More importantly than just that they are factors, NA is actually a level of X3. If it was a factor column, but NA was not a level, than in the conversion to numeric, it would not change into a 4, but it is a level (in fact the 4th level), so it becomes a 4. From ?factor here is the recommended way of converting factors to numeric. You see that this converts to a matrix properly
samp.frame <- data.frame(a = 1:10, b = factor(c(rep(1:3, each = 3), NA), exclude = NULL)) str(samp.frame) #Here NA becomes 4 samp.matrix <- data.matrix(samp.frame) samp.matrix #Convert the column in samp.frame first samp.frame$b <- as.numeric(levels(samp.frame$b))[samp.frame$b] str(samp.frame) #Now convert to a matrix samp.matrix <- data.matrix(samp.frame) samp.matrix I've never used the xlsx package, but an alternative to this process would be to save the file from Excel as a text file and then read it into R. That way you could control whether things were read in as factors or not. Cheers, Josh On Wed, Jul 14, 2010 at 7:58 AM, syrvn <ment...@gmx.net> wrote: > > Hi, > > I used str() on my data set: > > $ X1 : num 1 1 0 1 1 1 1 1 1 1 ... > $ X2 : num 0 1 0 2 1 2 0 2 2 0 ... > $ X3 : Factor w/ 4 levels "0","1","2","NA": 2 1 3 1 1 1 1 1 1 3 > ... > .... > > > The difference to your str() output is that in your case NA columns are > "num" columns > and in my case they are Factors. That's prob. why it replaces the NAs with 4 > after > applying data.matrix. > > I use the package xlsx to read the data in as an excel file. > > Cheers > -- > View this message in context: > http://r.789695.n4.nabble.com/convert-data-frame-to-matrix-NA-factor-problem-tp2288828p2288887.html > Sent from the R help mailing list archive at Nabble.com. > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > -- Joshua Wiley Ph.D. Student, Health Psychology University of California, Los Angeles http://www.joshuawiley.com/ ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
