do.expr: Python-Style Argument Unpacking

In ArcadeAntics/essentials: Essential Functions not Included in Base R

Python-Style Argument Unpacking

Description

Evaluate a call with Python-like argument unpacking, with or without names.

Usage

do.expr(expr, envir = parent.frame(),
              enclos = if (is.list(envir) || is.pairlist(envir))
                           parent.frame() else baseenv())

Arguments

expr	the (unevaluated) expression to be evaluated after unpacking its arguments.
envir, enclos	See eval; used to construct the environment in which the expression will be evaluated.

Details

In Python, an individual argument to a function may be unpacked into many arguments. The syntax looks something like:

⁠fun(*iter)⁠

where iter would be some type of iterable, normally a tuple or list. This is parsed to:

⁠fun(iter[0], iter[1], ..., iter[-2], iter[-1])⁠

Python has a similar syntax for unpacking named arguments:

⁠fun(**dict)⁠

where dict is a dictionary. This is parsed to:

⁠fun(dict.keys[0] = dict.values[0], dict.keys[1] = dict.values[1], ..., dict.keys[-2] = dict.values[-2], dict.keys[-1] = dict.values[-1])⁠

The syntax with do.expr is much the same:

⁠fun(`*`(list))⁠

is parsed to:

⁠fun(list[[1]], list[[2]], ...)⁠

and:

⁠fun(`**`(list))⁠

is parsed to:

⁠fun(names(list)[[1]] = list[[1]], names(list)[[2]] = list[[2]], ...)⁠

The object being unpacked does not have to be a list: any object with subsetting ([[), length, and names methods will suffice.

Value

The result of evaluating expr after unpacking its arguments.

Examples

x <- structure(1:5, names = letters[1:5])

do.expr( list(k = 1:4, `*`(x) ) )  # without names
do.expr( list(k = 1:4, `**`(x)) )  # with names

## use argument unpacking to select from multi-dimensional array
## (with unknown number of dimensions)

ndim <- sample(3:4, size = 1)
dim <- sample(3:4, size = ndim, replace = TRUE)
x <- array(seq_len(prod(dim)), dim)

## select rows whose sums are less than the mean row sum
sums <- rowSums(x)
i <- sums < mean(sums)
missing_indexes <- rep(list(quote(expr = )), length(dim(x)) - 1)
do.expr( x[i, `*`(missing_indexes)] )

## easy to add more arguments, such as 'drop'
do.expr( x[i, `*`(missing_indexes), drop = FALSE] )

ArcadeAntics/essentials documentation built on Nov. 7, 2024, 4:33 p.m.