Statistical Analysis: an Introduction using R/R/Lists

R is very particular about what can be contained in a vector. All the elements need to be of the same type, an moreover must be either types of number[1], logical values, or strings of text[2]. If you want a collection of elements which are of different types, or not of one of the allowed vector types, you need to use a list.
l1 <- list(a=1, b=1:3)
l2 <- c(sqrt, log) #

Notes edit

  1. There are actually 3 types of allowed numbers: "normal" numbers, complex numbers, and simple integers. This book deals almost exclusively with the first of these.
  2. This is not quite true, but unless you are a computer specialist, you are unlikely to use the final type: a vectors of elements storing "raw" computer bits, see ?raw