Technical description of tidyselect

In tidyselect: Select from a Set of Strings

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This is a technical description of the tidyselect syntax.

library(tidyselect)
library(magrittr)

# For better printing
mtcars <- tibble::as_tibble(mtcars)
iris <- tibble::as_tibble(iris)

To illustrate the semantics of tidyselect, we'll use variants of dplyr::select() and dplyr::rename() that return the named vector of locations for the selected or renamed elements:

select_loc <- function(data, ...) {
  eval_select(rlang::expr(c(...)), data)
}

rename_loc <- function(data, ...) {
  eval_rename(rlang::expr(c(...)), data)
}

Sets of variables

The tidyselect syntax is all about sets of variables, internally represented by integer vectors of locations. For example, c(1L, 2L) represents the set of the first and second variables, as does c(1L, 2L, 1L).

If a vector of locations contains duplicates, they are normally treated as the same element, since they represent sets. An exception to this occurs with named elements whose names differ. If the names don't match, they are treated as different elements in order to allow renaming a variable to multiple names (see section on Renaming variables).

Today, the syntax of tidyselect is generally designed around Boolean algebra, i.e. we recommend writing starts_with("a") & !ends_with("z"). Earlier versions of tidyselect had more of a flavour of set operations, so that you'd write starts_with("a") - ends_with("b"). While the set operations are still supported, and is how tidyselect represents variables internally, we no longer recommend them because Boolean algebra is easy for people to understand.

Bare names

Within data-expressions (see Evaluation section), bare names represent their own locations, i.e. a set of size 1. The following expressions are equivalent:

mtcars %>% select_loc(mpg:hp, !cyl, vs)

mtcars %>% select_loc(1:4, !2, 8)

The : operator

: can be used to select consecutive variables between two locations. It returns the corresponding sequence of locations.

mtcars %>% select_loc(2:4)

Because bare names represent their own locations, it is easy to select a range of variables:

mtcars %>% select_loc(cyl:hp)

Boolean operators

The | operator takes the union of two sets:

iris %>% select_loc(starts_with("Sepal") | ends_with("Width"))

The & operator takes the intersection of two sets:

iris %>% select_loc(starts_with("Sepal") & ends_with("Width"))

The ! operator takes the complement of a set:

iris %>% select_loc(!ends_with("Width"))

Taking the intersection with a complement produces a set difference:

iris %>% select_loc(starts_with("Sepal") & !ends_with("Width"))

Dots and c()

tidyselect functions can take dots, like dplyr::select(), or a named argument, like tidyr::pivot_longer(). In the latter case, the dots syntax is accessible via c(). In fact ... syntax is implemented through c(...) and is thus completely equivalent.

mtcars %>% select_loc(mpg, disp:hp)

mtcars %>% select_loc(c(mpg, disp:hp))

c(x, y, z) is a equivalent to x | y | z:

iris %>% select_loc(starts_with("Sepal"), ends_with("Width"), Species)

iris %>% select_loc(starts_with("Sepal") | ends_with("Width") | Species)

Renaming variables

Name combination and propagation

When named inputs are provided in ... or c(), the selection is renamed. If the inputs are already named, the outer and inner names are combined with a ... separator:

mtcars %>% select_loc(foo = c(bar = mpg, baz = cyl))

Otherwise the outer names is propagated to the selected elements according to the following rules:

• With data frames, a numeric suffix is appended because columns must be uniquely named.

  r mtcars %>% select_loc(foo = c(mpg, cyl))

• With normal vectors, the name is simply assigned to all selected inputs.

  r as.list(mtcars) %>% select_loc(foo = c(mpg, cyl))

Combination and propagation can be composed by using nested c():

mtcars %>% select_loc(foo = c(bar = c(mpg, cyl)))

Set combination with named variables

Named elements have special rules to determine their identities in a set. Unnamed elements match any names:

• a | c(foo = a) is equivalent to c(foo = a).
• a & c(foo = a) is equivalent to c(foo = a).

Named elements with different names are distinct:

• c(foo = a) & c(bar = a) is equivalent to c().
• c(foo = a) | c(bar = a) is equivalent to c(foo = a, bar = a).

Because unnamed elements match any named ones, it is possible to select multiple elements and rename one of them:

iris %>% select_loc(!Species, foo = Sepal.Width)

Predicate functions

Predicate function objects can be supplied as input in an env-expression, typically with the selection helper where(). They are applied to all elements of the data, and should return TRUE or FALSE to indicate inclusion. Predicates in env-expressions are effectively expanded to the set of variables that they represent:

iris %>% select_loc(where(is.numeric))

iris %>% select_loc(where(is.factor))

iris %>% select_loc(where(is.numeric) | where(is.factor))

iris %>% select_loc(where(is.numeric) & where(is.factor))

Selection helpers

We call selection helpers any function that inspects the currently active variables with peek_vars() and returns a selection.

• peek_vars() returns a character vector of names.
• The returned selection can be any output conforming to the types described in the Data types section.

Examples of selection helpers are all_of(), contains(), or last_col(). These selection helpers are evaluated as env-expressions (see Evaluation section).

Supported data types

The following data types can be returned from selection helpers or forced via !! or force() (the latter works in tidyselect because it is treated as an env-expression, see Evaluation section):

• Vectors of locations:

  r iris %>% select_loc(force(c(1, 3)))

• Vectors of names. These are matched and transformed to locations.

  r iris %>% select_loc(force(c("Sepal.Length", "Petal.Length")))

• Predicate functions. These are applied to all elements to determine inclusion.

  r iris %>% select_loc(force(is.numeric))

Evaluation

Data-expressions and env-expressions

tidyselect is not a typical tidy evaluation UI. The main difference is that there is no data masking. In a typical tidy eval function, expressions are evaluated with data-vars first in scope, followed by env-vars:

mask <- function(data, expr) {
  rlang::eval_tidy(rlang::enquo(expr), data)
}

foo <- 10
cyl <- 200

# `cyl` represents the data frame column here:
mtcars %>% mask(cyl * foo)

It is possible to bypass the data frame variables by forcing symbols to be looked up in the environment with !! or .env:

mtcars %>% mask(!!cyl * foo)
mtcars %>% mask(.env$cyl * foo)

With tidyselect, there is no such hierarchical data masking. Instead, expressions are evaluated either in the context of the data frame or in the user environment, without overlap. The scope of lookup depends on the kind of expression:

1. data-expressions are evaluated in the data frame only. This includes bare symbols, the boolean operators, -, :, and c(). You can't refer to environment-variables in a data-expression:

   r cyl_pos <- 2 mtcars %>% select_loc(mpg | cyl_pos)

2. env-expressions are evaluated in the environment. This includes all calls other than those mentioned above, as well as symbols that are part of those calls. You can't refer to data-variables in a data-expression:

   r mtcars %>% select_loc(all_of(mpg))

Because the scoping is unambiguous, you can safely refer to env-vars in an env-expression, without having to worry about potential naming clashes with data-vars:

x <- data.frame(x = 1:3, y = 4:6, z = 7:9)

# `ncol(x)` is an env-expression, so `x` represents the data frame in
# the environment rather than the column in the data frame
x %>% select_loc(2:ncol(x))

If you have variable names in a character vector, it is safe to refer to the env-var containing the names with all_of() because it is an env-expression:

y <- c("y", "z")
x %>% select_loc(all_of(y))

Note that currently, env-vars are still allowed in some data-expressions, for compatibility. However this is in the process of being deprecated and you should see a note recommending to use all_of() instead. This note will become a deprecation warning in the future, and then an error.

mtcars %>% select_loc(cyl_pos)

Arithmetic operators

Within data-expressions (see Evaluation section), +, * and / are overridden to cause an error. This is to prevent confusion stemming from normal data masking usage where variables can be transformed on the fly:

mtcars %>% select_loc(cyl^2)

mtcars %>% select_loc(mpg * wt)

Selecting versus renaming

The select and rename variants take the same types of inputs and have the same type of return value. They have a few important differences.

All renaming inputs must be named

Unlike eval_select() which can select without renaming, eval_rename() expects a fully named selection. If one or several names are missing, an error is thrown.

mtcars %>% select_loc(mpg)

mtcars %>% rename_loc(mpg)

Renaming to an existing variable name

If the input data is a data frame, tidyselect generally throws an error when duplicate column names are selected, in order to respect the invariant of unique column names.

# Lists can have duplicates
as.list(mtcars) %>% select_loc(foo = mpg, foo = cyl)

# Data frames cannot
mtcars %>% select_loc(foo = mpg, foo = cyl)

A selection can rename a variable to an existing name if the latter is not part of the selection:

mtcars %>% select_loc(cyl, cyl = mpg)

mtcars %>% select_loc(disp, cyl = mpg)

This is not possible when renaming.

mtcars %>% rename_loc(cyl, cyl = mpg)

mtcars %>% rename_loc(disp, cyl = mpg)

However, the name conflict can be solved by renaming the existing variable to another name:

mtcars %>% select_loc(foo = cyl, cyl = mpg)

mtcars %>% rename_loc(foo = cyl, cyl = mpg)

Duplicate columns in data frames

Normally a data frame shouldn't have duplicate names. However, the real world is messy and duplicates do happen in the wild. tidyselect tries to be as permissive as it can with these duplicates so that users can restore unique names with select() or rename().

First let's create a data frame with duplicate names:

dups <- vctrs::new_data_frame(list(x = 1, y = 2, x = 3))

If the duplicates are not part of the selection, they are simply ignored:

dups %>% select_loc(y)

If the duplicates are selected, this is an error:

dups %>% select_loc(x)

The duplicate names can be repaired by renaming chosen locations:

dups %>% select_loc(x, foo = 3)

dups %>% rename_loc(foo = 3)

Acknowledgements

The tidyselect syntax was inspired by the base::subset() function written by Peter Dalgaard. The select parameter of subset.data.frame() is evaluated in a data mask where the column names are bound to their locations in the data frame. This allows : to create sequences of variable locations. The locations can be combined with c(). This selection interface set the tone for the development of the tidyselect syntax.

mtcars %>% subset(select = c(cyl, hp:wt))

tidyselect documentation built on Oct. 11, 2022, 1:07 a.m.