Question

Why do we have to use data(spam) before we can run lda on it ? spam is a dataset in the ElemsStatLearn package.

library(ElemStatLearn)
library(MASS) # for lda
spam[5] # is ok
spam.lda = lda(spam ~ . , data = spam) # not ok
data(spam)
spam.lda = lda(spam ~ . , data = spam) # ok 

We can access spam[5] even before we run data(spam).

Answer 1

R includes a range of datasets, mainly for use in examples. Some of these datasets are directly available (e.g. iris or cars), in some packages they are not automatically available requiring a call to data to attach them to the current workspace, e.g. meuse from the gstat package. So you experience in ElemStatLearn is valid, spam is automatically available.