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.