Question

I know this is a basic question but for some strange reason I am unable to find an answer.

How should I apply basic statistical functions like mean, median, etc. over entire array, matrix or dataframe to get unique answers and not a vector over rows or columns

Answer 1

Since this comes up a fair bit, I’m going to treat this a little more comprehensively, to include the ‘etc.’ piece in addition to mean and median.

1. For a matrix, or array, as the others have stated, mean and median will return a single value. However, var will compute the covariances between the columns of a two dimensional matrix. Interestingly, for a multi-dimensional array, var goes back to returning a single value. sd on a 2-d matrix will work, but is deprecated, returning the standard deviation of the columns. Even better, mad returns a single value on a 2-d matrix and a multi-dimensional array. If you want a single value returned, the safest route is to coerce using as.vector() first. Having fun yet?

2. For a data.frame, mean is deprecated, but will again act on the columns separately. median requires that you coerce to a vector first, or unlist. As before, var will return the covariances, and sd is again deprecated but will return the standard deviation of the columns. mad requires that you coerce to a vector or unlist. In general for a data.frame if you want something to act on all values, you generally will just unlist it first.

Edit: Late breaking news(): In R 3.0.0 mean.data.frame is defunctified:

o   mean() for data frames and sd() for data frames and matrices are
defunct.