Index vectors

In the exercises below we cover the basics of index vectors. Before proceeding, first read section 2.7 of An Introduction to R, and the help pages for the sum, and which functions.

Answers to the exercises are available here.

Exercise 1
If x <- c("ww", "ee", "ff", "uu", "kk"), what will be the output for x[(c(2,3)]?
a. "ee", "ff"
b. "ee"
c. "ff"

Exercise 2
If x <- c("ss", "aa", "ff", "kk", "bb"), what will be the third value in the index vector operation x?
a. "uu"
b. NA
c. "kk"

Exercise 3
If x <- c("pp", "aa", "gg", "kk", "bb"), what will be the fourth value in the index vector operation x[-2]?
a. "aa"
b. "gg"
c. "bb"

Exercise 4
Let a <- c(2, 4, 6, 8) and b <- c(TRUE, FALSE, TRUE, FALSE), what will be the output for the R expression max(a[b])?

Exercise 5
Let a <- c (3, 4, 7, 8) and b <- c(TRUE, TRUE, FALSE, FALSE), what will be the output for the R expression sum(a[b])?

Exercise 6
Write an R expression that will return the sum value of 10 for the vector x <- c(2, 1, 4, 2, 1, NA)

Exercise 7
If x <- c(1, 3, 5, 7, NA) write an r expression that will return the output 1, 3, 5, 7.

Exercise 8
Consider the data frame s <- data.frame(first= as.factor(c("x", "y", "a", "b", "x", "z")), second=c(2, 4, 6, 8, 10, 12)). Write an R statement that will return the output 2, 4, 10, by using the variable first as an index vector.

Exercise 9
What will be the output for the R expression (c(FALSE, TRUE)) || (c(TRUE, TRUE))?

Exercise 10
Write an R expression that will return the positions of 3 and 7 in the vector x <- c(1, 3, 6, 7, 3, 7, 8, 9, 3, 7, 2).