Calculate correlation by aggregating columns of data frame

Question

I have the following data frame:

y <- data.frame(group = letters[1:5], a = rnorm(5) , b = rnorm(5), c = rnorm(5), d = rnorm(5) )

How to

Answer 1

You can use apply to apply a function to each row (or column) of a matrix, array or data.frame.

apply(
  y[,-1], # Remove the first column, to ensure that u remains numeric
  1,      # Apply the function on each row
  function(u) cor( u[1:2], u[3:4] )
)

(With just 2 observations, the correlation can only be +1 or -1.)