On Thursday Problem Set 2 was distributed. A digital version of the prompt can be accessed here: ProblemSet2.pdf. The highlights from this week are summarized below:
To extract individual elements or sub-vectors from a vector the
 notation can be used. Numbers (integers) supplied inside of the
 will instruct R to extract the objects located at the corresponding position inside of the vector.
# extracts the element at position 26 of the vector letters # will return "z" letters # will return "h" letters # will return vector containing "h" and "z" letters[c(8, 26)] # will return vector containing "a", "b", and "c" letters[1:3] # will return letter "i" letters[8+1] # will return "d" Fourth_letter <- 4 letters[Fourth_letter] # will return vector of length 24 containing # all letters except "a" and "h" letters[-c(1,8)] # assignes the first three letters to a # new vector called x x <- letters[1:3] # replaces the second element of vector x # with the word "Hello" x <- "Hello"
We also discussed the
which() function which returns a vector of index locations meeting a logical condition and the
subset() function which can be used to extract elements meeting a certain condition.
# evaluates to 8 and 17 which(letters == "h" | letters == "q") # extract letters h and q subset(x = letters, letters == "h" | letters == "q")
Indexing w/ TRUE and FALSE
Among the things covered was the use of TRUE and FALSE inside of the
 brackets to extract elements from vectors.
abc <- c("a", "b", "c", "d", "e") # returns a vector containing all the elements of abc abc[c(TRUE, TRUE, TRUE, TRUE, TRUE)] # returns a vector that is empty (character(0)) abc[c(FALSE, FALSE, FALSE, FALSE, FALSE)] # the empty vector is of type character typeof(x = abc[c(FALSE, FALSE, FALSE, FALSE, FALSE)]) # the empty vector is indeed empty and has length 0 length(x = abc[c(FALSE, FALSE, FALSE, FALSE, FALSE)]) # returns a vector containing only the second and forth element of abc # that is "b" and "d" abc[c(FALSE, TRUE, FALSE, TRUE, FALSE)] # creates an empty vector of type double foo <- c(pi, -0.5)[c(FALSE, FALSE)] # as does this foobar <- pi[FALSE]
With this in mind, we can use logical arguments to subset vectors.
# returns a vector of length 26 # containing the logical value FALSE 24 times and # the logical value TRUE 2 times (in the first and # last position) letters == "a" | letters == "z" # returns the letters "a" and "z" letters[letters == "a" | letters == "z"]
Managing Objects and Data
R is perfectly capable of serving as a file-system browser.
# the present working directory can be identified via getwd() # files and folders present in the current working # directory can be identified via list.files() # files and folders present in an arbitrary # directory can be identified by specifying the path # for example: list.files(path = "C:/Users/UserName/Desktop") # windows list.files(path = "/Users/Username/Desktop") # mac # the working directory can be changed via setwd(dir = "C:/Users/UserName/Desktop") # windows setwd(dir = "/Users/Username/Desktop") # mac # saving objects stored in R's active memory # in the current working directory (with the .Rdata extension) x <- rnorm(n = 100, mean = 0, sd = 10) y <- runif(n = 55, min = 0, max = 1) save(x, y, file = "my-data.RData") # removes all objects in R's active memory rm(list = ls()) # load the objects x and y back into R's active memory load(file = "my-data.RData")