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? ::: {.cell}

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? ::: {.cell}

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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? ::: {.cell}

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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? ::: {.cell}

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

15. What does ncol(df) return for a data frame df? ::: {.cell}

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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? ::: {.cell}

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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? ::: {.cell}

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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? ::: {.cell}

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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? ::: {.cell}

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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? ::: {.cell}

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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? ::: {.cell}

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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? ::: {.cell}

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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? ::: {.cell}

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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? ::: {.cell}

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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? ::: {.cell}

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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? ::: {.cell}

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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? ::: {.cell}

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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? ::: {.cell}

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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? ::: {.cell}

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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? ::: {.cell}

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

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