[R] Translation of R code required

[Barry Rowlingson]

Sarah Goslee wrote:

> Since this step works,
> 
> mental$Rx <- factor(mental$Rx, levels=c("VS","IPS"))
> 
> I think that some of the names are right, but perhaps that
> one is spelled differently (Centre vs centre, maybe, since
> R is case-sensitive?).

  How do you know that step works? If the dataframe 'mental' doesnt have 
an 'Rx' column then mental$Rx will be 'NULL', and the factor() function 
will make a factor out of that...

  > x=data.frame(foo=1:10)

  so x is a data frame with one column, called 'foo'. Now lets make 
levels from a non-existent column, 'bar':

  > x$bar=factor(x$bar,levels=c('x','y'))

  No complaints... But try printing 'x' now (by typing x at the command 
line)... Ick!

  Try things one line at a time and check the objects created or 
modified are sensible.

Barry