Recursively apply a function over an expression, or any expression elements of a list, and optionally the subexpressions within any expressions.
Usage
expr_apply(
x,
f,
depth = Inf,
into = FALSE,
order = c("pre", "post"),
how = c("replace", "unlist", "unique"),
env = parent.frame()
)Arguments
- x
The R object; can an expression, or a list of arbitrary nestedness potentially containing expressions.
- f
Function to apply to all expressions within
x; takes 1 to 3 arguments.- depth
How many levels to recurse into lists; default is
Inf.- into
Whether to recurse into expressions. Can be
TRUEto visit all subexpressions,FALSEto not recurse, or"leaves"to recurse and only applyfto terminal nodes of expressions (i.e. the symbols and syntactic literals comprising the expressions).- order
Whether a parent node is visited before ("pre") or after ("post") its children (the terminology comes from pre-order and post-order depth-first search). This only has an effect if
into == TRUE.- how
How to structure the result.
- env
Environment for injections in
x(see expression).
Value
If how = "replace" (the default), the original object x with f
applied to expressions within it. If how = "unlist", the same but with
unlist() applied to it. If how = "unique", first unlist() then
unique() are applied.
Details
The function f can take one to three arguments. The first argument is the
expression itself for f to apply to, and f should return some kind of
replacement for, or modified version of, this argument.
The second argument is a list with information about the name of the
expression in the list x and of its parents. Specifically, the first
element of the list is the name of the expression, the second element of the
list is the name of the "parent" of the expression, and so on. If any
elements in this chain are unnamed, an integer is provided as the name. If
the expression is within another expression (which only happens with
into = TRUE), this is signalled as a NULL at the top of the list, one
for each level of recursion into the expression.
The third argument is an integer vector, the index into x where
f is currently operating. This is suitable for use with expr_sub().