eval_relocate {tidyselect}

Evaluate an expression to relocate variables

Description

eval_relocate() is a variant of eval_select() that moves a selection to a new location. Either before or after can be provided to specify where to move the selection to. This powers dplyr::relocate().

Usage

eval_relocate(
  expr,
  data,
  ...,
  before = NULL,
  after = NULL,
  strict = TRUE,
  name_spec = NULL,
  allow_rename = TRUE,
  allow_empty = TRUE,
  allow_predicates = TRUE,
  before_arg = "before",
  after_arg = "after",
  env = caller_env(),
  error_call = caller_env()
)

Arguments

expr

Defused R code describing a selection according to the tidyselect syntax.

data

A named list, data frame, or atomic vector. Technically, data can be any vector with names() and "[[" implementations.

...

These dots are for future extensions and must be empty.

before, after

Defused R code describing a selection according to the tidyselect syntax. The selection represents the destination of the selection provided through expr. Supplying neither of these will move the selection to the left-hand side. Supplying both of these is an error.

strict

If TRUE, out-of-bounds errors are thrown if expr attempts to select or rename a variable that doesn't exist. If FALSE, failed selections or renamings are ignored.

name_spec

A name specification describing how to combine or propagate names. This is used only in case nested c() expressions like c(foo = c(bar = starts_with("foo"))). See the name_spec argument of vctrs::vec_c() for a description of valid name specs.

allow_rename

If TRUE (the default), the renaming syntax c(foo = bar) is allowed. If FALSE, it causes an error. This is useful to implement purely selective behaviour.

allow_empty

If TRUE (the default), it is ok for expr to result in an empty selection. If FALSE, will error if expr yields an empty selection.

allow_predicates

If TRUE (the default), it is ok for expr to use predicates (i.e. in where()). If FALSE, will error if expr uses a predicate. Will automatically be set to FALSE if data does not support predicates (as determined by tidyselect_data_has_predicates()).

before_arg, after_arg

Argument names for before and after. These are used in error messages.

env

The environment in which to evaluate expr. Discarded if expr is a quosure.

error_call

The execution environment of a currently running function, e.g. caller_env(). The function will be mentioned in error messages as the source of the error. See the call argument of abort() for more information.

Value

A named vector of numeric locations with length equal to length(data). Each position in data will be represented exactly once.

The names are normally the same as in the input data, except when the user supplied named selections with c(). In the latter case, the names reflect the new names chosen by the user.

Examples

library(rlang)

# Interpret defused code as a request to relocate
x <- expr(c(mpg, disp))
after <- expr(wt)
eval_relocate(x, mtcars, after = after)

# Supplying neither `before` nor `after` will move the selection to the
# left-hand side
eval_relocate(x, mtcars)

# Within a function, use `enquo()` to defuse a single argument.
# Note that `before` and `after` must also be defused with `enquo()`.
my_relocator <- function(x, expr, before = NULL, after = NULL) {
  eval_relocate(enquo(expr), x, before = enquo(before), after = enquo(after))
}

my_relocator(mtcars, vs, before = hp)

# Here is an example of using `eval_relocate()` to implement `relocate()`.
# Note that the dots are passed on as a defused call to `c(...)`.
relocate <- function(.x, ..., .before = NULL, .after = NULL) {
  pos <- eval_relocate(
    expr(c(...)),
    .x,
    before = enquo(.before),
    after = enquo(.after)
  )
  set_names(.x[pos], names(pos))
}

relocate(mtcars, vs, .before = hp)
relocate(mtcars, starts_with("d"), .after = last_col())