rapply Recursively Apply a Function to a List

Description

rapply is a recursive version of lapply with flexibility in how the result is structured (how = "..").

Usage

rapply(object, f, classes = "ANY", deflt = NULL,
       how = c("unlist", "replace", "list"), ...)

Arguments

object	a list or expression, i.e., “list-like”.
f	a function of one “principal” argument, passing further arguments via ....
classes	character vector of class names, or "ANY" to match any class.
deflt	The default result (not used if how = "replace").
how	character string partially matching the three possibilities given: see ‘Details’.
...	additional arguments passed to the call to f.

Details

This function has two basic modes. If how = "replace", each element of object which is not itself list-like and has a class included in classes is replaced by the result of applying f to the element.

Otherwise, with mode how = "list" or how = "unlist", conceptually object is copied, all non-list elements which have a class included in classes are replaced by the result of applying f to the element and all others are replaced by deflt. Finally, if how = "unlist", unlist(recursive = TRUE) is called on the result.

The semantics differ in detail from lapply: in particular the arguments are evaluated before calling the C code.

In R 3.5.x and earlier, object was required to be a list, which was not the case for its list-like components.

Value

If how = "unlist", a vector, otherwise “list-like” of similar structure as object.

References

Chambers, J. A. (1998) Programming with Data. Springer.
(rapply is only described briefly there.)

See Also

lapply, dendrapply.

Examples

X <- list(list(a = pi, b = list(c = 1L)), d = "a test")
# the "identity operation":
rapply(X, function(x) x, how = "replace") -> X.; stopifnot(identical(X, X.))
rapply(X, sqrt, classes = "numeric", how = "replace")
rapply(X, deparse, control = "all") # passing extras. argument of deparse()
rapply(X, nchar, classes = "character", deflt = NA_integer_, how = "list")
rapply(X, nchar, classes = "character", deflt = NA_integer_, how = "unlist")
rapply(X, nchar, classes = "character",                      how = "unlist")
rapply(X, log, classes = "numeric", how = "replace", base = 2)

## with expression() / list():
E  <- expression(list(a = pi, b = expression(c = C1 * C2)), d = "a test")
LE <- list(expression(a = pi, b = expression(c = C1 * C2)), d = "a test")
rapply(E, nchar, how="replace") # "expression(c = C1 * C2)" are 23 chars
rapply(E, nchar, classes = "character", deflt = NA_integer_, how = "unlist")
rapply(LE, as.character) # a "pi" | b1 "expression" | b2 "C1 * C2" ..
rapply(LE, nchar)        # (see above)
stopifnot(exprs = {
  identical(E , rapply(E , identity, how = "replace"))
  identical(LE, rapply(LE, identity, how = "replace"))
})