Which is the fastest way to calculate groupwise means of a variable "x" for each level of a factor called "factor", given you have installed base R, data.table, and plyr?

matrix(1:4, ncol = 2)[1,]

Suppose we have defined a list object, "X", which has 3 components. Each component is a vector with at least 5 elements. Which command will extract the fourth (4th) element from the third (3rd) component of our list?

What is the output of the following calculation? t(c(1, 1)) %*% c(1, 1)

Let X be a n-dimension vector. Which call tests this particular statement: "no value of X is NA"

Arrays are similar to matrices in R, but are different in what respect?

What is the output of matrix(1:10, nrow=2)[2,2]?

Which class avoids making copies of objects?

Given x <- c(1:3,NA), the command: (x+1)[(!is.na(x)) & x>0] -> z generates:

Which is an invalid way to return the mean of each variable in a numeric data frame?

Which is NOT the parameter of command seq()?