As of R version 3.0.0 one cannot use functions such as mean(), max(),
min(), etc., on data.frame rows. This can be a problem
when going through material in the Creating functions lesson,
Testing and Documenting section, if one attempts to apply any of these
functions to the example dataset rows (e.g., center(dat[4, ], 0). The
examples in the lesson are all written to work only with columns, but
an error is returned if one tries using rows. This may also be a problem
if one attempts to calculate row means or medians in the
Analyzing patient data lesson,
Manipulating Data section.
The recommended solution to this issue is to not apply these functions
to data.frame rows. However, one could get around this issue by
using the functions rowSums or rowMeans in some cases, or
explicitly converting the row to a vector of numeric values
(e.g., center(as.numeric(dat[4, ]), 0))