Multiple Choices: Data Frames

Questions

1. What function is used to create a data frame in R?

matrix() cbind() data.frame() df()

2. If df <- data.frame(A = 1:3, B = c("X", "Y", "Z")), what does df$B return?

A vector with elements "X", "Y", "Z" A data frame with column B A list with elements "X", "Y", "Z" An error

3. What is the output of nrow(data.frame(x = 1:4, y = 5:8))?

8 2 4 An error

4. Given df <- data.frame(Name = c("Alice", "Bob"), Age = c(25, 30)), how would you select the first row of the data frame?

df[1, ] df[, 1] df[1] df[[1]]

5. If df <- data.frame(A = 1:5, B = c(NA, 2, NA, 4, 5)), what will na.omit(df) do?

Remove the columns with NA values Remove rows containing NA values Replace NA values with zeros Return the number of NA values

6. If df <- data.frame(x = 1:5, y = c(10, 20, 30, 40, 50)), what does df[ , "y"] return?

The first row of df The y column as a vector A data frame containing only column y The sum of y

7. What does str(df) display for a data frame df?

The structure of the data frame, including column types and a preview of values A summary of each column’s mean and median Only the dimensions of df A list of data frame names

8. Given df <- data.frame(A = c(5, 3, 2, 8), B = c("A", "B", "C", "D")), what will df[order(df$A), ] return?

The data frame sorted by column B The data frame sorted by column A Only column A sorted in ascending order An error

9. Which of the following will add a new column C with values c(100, 200, 300, 400) to df <- data.frame(A = 1:4, B = 5:8)?

df <- data.frame(df, C = c(100, 200, 300, 400)) df <- rbind(df, c(100, 200, 300, 400)) df$C <- c(100, 200, 300, 400) df <- cbind(df, c(100, 200, 300, 400))

10. If df <- data.frame(Name = c("Alice", "Bob", "Carol"), Age = c(25, NA, 30)), what does is.na(df$Age) return?

The total count of NA values in Age A logical vector indicating TRUE for missing values in Age The non-missing values in Age An error

11. Which function would you use to view the first few rows of a data frame df?

head(df) tail(df) first(df) preview(df)

12. Given df <- data.frame(A = c(2, 4, 6), B = c("X", "Y", "Z")), what will df[1:2, ] return?

The first two rows of df The first two columns of df The first two elements of each column An error

13. If df <- data.frame(A = 1:3, B = 4:6), which command will rename the columns to X and Y?

colnames(df) <- c("X", "Y") rownames(df) <- c("X", "Y") df <- rename(df, X, Y) df <- colnames(df, c("X", "Y"))

14. Which function gives a summary of each column in a data frame df, including min, max, mean, and quartiles?

describe(df) str(df) summary(df) info(df)

15. What does ncol(df) return for a data frame df?

The number of rows The number of columns The dimensions of df A data frame with the number of columns

16. Given df1 <- data.frame(A = 1:3, B = 4:6) and df2 <- data.frame(A = 7:9, B = 10:12), what does rbind(df1, df2) do?

Adds df2 as new columns to df1 Adds df2 as new rows to df1 Combines df1 and df2 element-wise Returns an error

17. How would you select only the columns B and C from a data frame df?

df[c("B", "C")] df[, c("B", "C")] df[ , c(B, C)] df[[c("B", "C")]]

18. If df <- data.frame(A = 1:5, B = 6:10), which of the following commands will add a new row c(11, 12) to df?

rbind(df, c(11, 12)) df[, c("B", "C")] df <- data.frame(df, c(11, 12)) df <- cbind(df, c(11, 12))

19. Which function checks if an object df is a data frame?

check.data.frame(df) data.frame.check(df) is.data.frame(df) df.is.data.frame()

20. If df <- data.frame(A = c("apple", "banana", "apple"), B = c(5, 3, 8)), which command will count the number of occurrences of each unique value in column A?

table(df$A) count(df$A) sum(df$A) unique(df$A)

21. Given df <- data.frame(A = c(3, 1, 2), B = c(9, 6, 4)), what does df[order(df$A), ] return?

The data frame sorted by column B The data frame sorted by column A in ascending order The data frame sorted by column A in descending order An error

22. If df <- data.frame(X = c(1, NA, 3, 4), Y = c("A", "B", NA, "D")), which function would return a logical vector indicating missing values in X?

is.null(df$X) is.na(df$X) sum(df$X) which(df$X)

23. Which function can you use to combine two data frames df1 and df2 with the same columns, adding df2 as rows to df1?

cbind(df1, df2) merge(df1, df2) rbind(df1, df2) combine(df1, df2)

24. If df <- data.frame(A = 1:5, B = letters[1:5]), what will df[ , 1] return?

The first row as a vector The first column as a vector The entire data frame An error

25. Given df <- data.frame(A = c(1, 2, 3, 4), B = c(10, 20, 30, 40)), which command will select rows where column B is greater than 20?

df[df$B > 20, ] df[ , df$B > 20] df[ , B > 20] df[20 < B, ]

26. Which function will display the names of columns in a data frame df?

names(df) columns(df) colnames(df) Both A and C

27. If df <- data.frame(A = 1:5, B = 6:10), what does df[-1, ] return?

The first row of df All columns except the first All rows except the first The data frame without column A

28. To find the unique values in a column A of a data frame df, which command would you use?

table(df$A) unique(df$A) is.unique(df$A) distinct(df$A)

29. If df <- data.frame(A = 1:3, B = 4:6), what does dim(df) return?

The number of rows only The number of columns only A vector with the number of rows and columns A data frame of dimensions

30. Given df <- data.frame(A = c(1, 2, 3), B = c("X", "Y", "Z")), how would you convert column A to a character type?

df$A <- as.character(A) df$A <- as.character(df$A) as.character(df$A) convert(df$A, "character")