Question 1.

In R, suppose points = data.frame(X=1:4, Y=c(100, 80, 120, 90)). What is the value returned from points[,1]?

• a data frame representing the point (X=1, Y=100)
• a vector of integers representing x-coordinates of the 4 points
• a vector of integers representing y-coordinates of the 4 points
• an integer representing x-coordinate of the point (X=1, Y=100)
• an integer representing y-coordinate of the point (X=1, Y=100)

Question 2.

In R, suppose data = data.frame(brand=c('A', 'B', 'C', 'D'), sales=c(100, 80, 120, 90)). We like to query the brands whose sales are greater than 90. Which of the following expression returns a list of such brand names?

• as.list(subset(data, data$sales > 90))
• as.list(data[data$sales > 90,][0])
• as.list(data[data$sales > 90,][1])
• as.list(data[data$sales > 90,])

Those who chose the first answer might have never heard of using logical vector for indesing. They thus picked a seemingly safer answer.

Those who chose the second answer thought indexing starts from 0 like many other programming languages do.

Those who chose the fourth answer thought logical indexing for selecting rows is symmetric with column selection by reversing the posistions.