R

Summary statistics by two or more factor variables

calsalmon June 9, 2013

How about this, still using tapply()? It’s more versatile than you knew!

with(mtcars, tapply(mpg, list(cyl, gear), sum))
#       three  four five
# four   21.5 215.4 56.4
# six    39.5  79.0 19.7
# eight 180.6    NA 30.8

Or, if you’d like the printed output to be a bit more interpretable:

with(mtcars, tapply(mpg, list("Cylinder#"=cyl, "Gear#"=gear), sum))

If you want to use more than two cross-classifying variables, the idea’s exactly the same. The results will then be returned in a 3-or-more-dimensional array:

A <- with(mtcars, tapply(mpg, list(cyl, gear, carb), sum))

dim(A)
# [1] 3 3 6
lapply(1:6, function(i) A[,,i]) # To convert results to a list of matrices

# But eventually, the curse of dimensionality will begin to kick in...
table(is.na(A))
# FALSE  TRUE 
#    12    42