S-PLUS Lecture 2 Jaeyong Lee Department of Statistics Pennsylvania State University

Naming Conventions Names in S-Plus are made up of upper- and lower-case alphabets, 0-9, and period, ‘.’, in any noninitial position. S is case sensitive. The underscore, ‘_’, is not available.

Language Layout S-Plus commands are either expressions or assignments. 1 – pi +exp(1.7) a <- 6 Commands are separated by either ‘;’ or a newline. a <- 7; b <- a*7 ‘#’ marks the rest of line as a comment. a + b # This is an illustration. If a command is not complete, S-Plus issues ‘+’ rather than ‘>’. ‘.Last.value’ stores the most recently evaluated non-assignment expression.

Language Layout-Continued If a command is not complete, S-Plus issues ‘+’ rather than ‘>’. a * exp(3 +.Last.value stores the most recently evaluated non- assignment expression. 1 – pi + exp(1.7).Last.value a <- 6.Last.value

Vectors In S-Plus, there are no scalars; vectors of length 1 are used instead. Vectors are made up of numeric, logical values, or character strings. But you can not mix them. Character strings can be entered with either double or single quotes, but will always be printed with double quotes.

Session: Vectors x <- c(1.2, 2.4, 2.7, 5.6, 7.2) colours <- c(“red”, “green”, “blue”, “white”) x[3] colours[2] x > 3 names(x) <- c(‘a’,’b’,’c’,’d’,’e’) x x[b] x[“b’] letters x[letters[1:3]] x[-(3:5)]

Vector Arithmetic Arithmetic operations on vectors are performed element by element. If two vectors in the same expression have different length, the expression will produce a vector with the same length as the longest in the expression. The shorter vectors are recycled until they match the length of the longest.

Session: Vector Arithmetic x <- 1:20 y <- 1:7 2*x sin(x) x+2y :1 seq(from=10, to = 1, by=.5) rep(x, times=3)

Logical Vectors Possible elements of a logical vector is T (True) and F (False). Logical operators are >, >=, <, <=, ==, !=. c1&c2 (intersection), c1|c2 (union), !c1 (negation). A logical vector can be used in ordinary arithmetic, in which case F and T become 0 and 1, respectively.

Session: Logical Vectors x <- 1:30 big 20 medium =10) & (x <=20) sum(x < 10)

Missing Value Marker, NA When an element of a vector is “not available” or “missing”, the special value NA can be used to reserve the place. In general, any operation on an NA becomes an NA.

Session: NA x <- 1:5 is.na(x) x[3] <- NA is.na(x)

Session: Character Vector X <- c(“abc”, “def”, “ghi”) paste(“abc”, “def) paste(“abc”, “def”, sep=“”) paste(c(“X”,”Y”),1:10,sep=“”)

Index Vectors Subsets of elements of a vector can by selected by appending to a name of the vector an index vector in square brackets. An index expression can appear on the receiving side of an assignment expression. There are four kinds of index vectors: a logical vector (in this case, the lengths of the vector and index vector must match.) a positive integral vector a negative integral vector a character string vector.

Intrinsic Attributes: mode and length Every object in S-Plus has two attributes, mode and length. The mode of a vector can be numeric, complex, logical, and character. The function attributes(x) gives all non-intrinsic attributes of x. There is a special attribute called class which was introduced to enhance object-oriented programming. For example, plot() will react a different class differently.

Session: Intrinsic Attributes x <- 1:10 mode(x) length(x) attributes(x) dim(x) <- c(2,5) attributes(x) dim(x) <- c(10) digits <- as.character(x) y <- as.numeric(digits) E <- numeric() E[4] <- 17

Session: Intrinsic Attributes x <- 1:10 mode(x) length(x) attributes(x) dim(x) <- c(2,5) attributes(x) dim(x) <- c(10) digits <- as.character(x) y <- as.numeric(digits) E <- numeric() E[4] <- 17 leukemia mode(leukemia) length(leukemia) attributes(leukemia) unclass(leukemia)