[R] using the function unique(), but asking it to ignore a column of a data.frame

[Andrew Yee]

Take for example the following data.frame:

a<-c(1,1,5)
b<-c(3,2,3)
c<-c(5,1,5)
sample.data.frame<-data.frame(a=a,b=b,c=c)

I'd like to be able to use unique(sample.data.frame), but have
unique() ignore column a when determining the unique elements.

However, I figured that this would be setting for incomparables=, but
it appears that this funcationality hasn't been incorporated.  Is
there a work around for this, i.e. to be able to get unique to only
look at selected columns of a data frame?

Thanks,
Andrew