There seems to be general agreement that l in "lapply" means a list, s in "sapply" means simplification, and r in "rapply" means recursively. But I could not find anything on t in "tapply". I'm very curious right now.
Racks for table , because tapply is a common form of table function. This can be seen by comparing the following calls:
table
tapply
x <- sample(letters, 100, rep=T) table(x) tapply(x, x, length)
although obviously tapply can do more than count.
In addition, some links related to "table-apply":
I think of it as "table" -apply, since the result comes as a matrix / table, and its dimensions are set by the INDEX arguments.
Another somewhat similar feature is xtabs.
tapply is an odd person. As far as I know, and as regards the R documentation for the functions that are used, the “t” is worth nothing, unlike the other functions that are used, which indicate input or output parameters.