by2: Apply a Function to Data by Group
In quest: Prepare Questionnaire Data for Analysis

Description Usage Arguments Value See Also Examples

by2 applies a function to data by group and is an alternative to the base R function by. The function is apart of the split-apply-combine type of function discussed in the plyr R package and is very similar to dlply. It splits up one data.frame .data[.vrb.nm]into a data.frame for each group in .data[.grp.nm], applies a function .fun to each data.frame, and then returns the results as a list with names equal to the group values unique(interaction(.data[.grp.nm], sep = .sep)). by2 is simply split.data.frame + lapply. Similar to dlply, The arguments all start with . so that they do not conflict with arguments from the function .fun. If you want to apply a function a (atomic) vector rather than data.frame, then use tapply2.

1	by2(.data, .vrb.nm, .grp.nm, .sep = ".", .fun, ...)

`.data`	data.frame of data.
`.vrb.nm`	character vector specifying the colnames of `.data` to select the set of variables to apply `.fun` to.
`.grp.nm`	character vector specifying the colnames of `.data` to select the grouping variables.
`.sep`	character vector of length 1 specifying the string to combine the group values together with. `.sep` is only used if there are multiple grouping variables (i.e., `length(.grp.nm)` > 1).
`.fun`	function to apply to the set of variables `.data[.vrb.nm]` for each group.
`...`	additional named arguments to pass to `.fun`.

list of objects containing the return object of .fun for each group. The names are the unique combinations of the grouping variables (i.e., unique(interaction(.data[.grp.nm], sep = .sep))).

by tapply2 dlply

# one grouping variable
by2(mtcars, .vrb.nm = c("mpg","cyl","disp"), .grp.nm = "vs",
   .fun = cov, use = "complete.obs")

# two grouping variables
x <- by2(mtcars, .vrb.nm = c("mpg","cyl","disp"), .grp.nm = c("vs","am"),
   .fun = cov, use = "complete.obs")
print(x)
str(x)

# compare to by
vrb_nm <- c("mpg","cyl","disp") # Roxygen runs the whole script if I put a c() in a []
grp_nm <- c("vs","am") # Roxygen runs the whole script if I put a c() in a []
y <- by(mtcars[vrb_nm], INDICES = mtcars[grp_nm],
   FUN = cov, use = "complete.obs", simplify = FALSE)
str(y) # has dimnames rather than names