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list

For representing a list of arbitrary objects

Create

Action	Code	Details
Empty list	list()
List with single element	list('hello')
Define list	list(1, 'a', third = c(1, 2))
List of NULL values, length n	vector('list', length = n)
List of repeated scalar value v, repeated n times	rep(v, n) |> as.list()	Only works for scalar (length 1) input
List of repeated object obj, repeated n times	replicate(n, obj, simplify = FALSE)	Also works for vector objects
From vector	as.list(vec)
From string, character per element	strsplit('hello', '')[[1]] |> as.list()
From data.frame, as a named list of column vectors	as.list(df)
From data.frame, as a list of row vectors	split(df, seq(nrow(df)))
From environment, as name-value pairs	as.list(env)

Test

Action	Code	Details
Is list or subclass	is.list(x)	Also TRUE for data.frame and other list-based data structures
Is list and not subclass	is(x, 'list')
Empty	length(x) == 0
Not empty	length(x) > 0
Not empty	assertthat::not_empty(x)
Pairwise equal to another list	identical(x, y)
Pairwise equal to another list	all.equal(x, y) |> isTRUE()
Contains NULL	sapply(x, is.null) |> any()
Contains NULL	vapply(x, is.null, FUN.VALUE = TRUE) |> any()	Fastest
Contains NULL	list(NULL) %in% x	Slowest
Contains element e	e %in% x
Contains specific scalar elements	all(c(1, 2) %in% x)
Contains specific objects	all(list(1, 2) %in% x)
Does not contain element e	not(e %in% x)
Elements are all of type t	sapply(x, is.numeric) |> all()
No duplicate elements	len(x) == len(unique(x))
Has duplicate elements	len(x) != len(unique(x))

extract

Action	Code	Details
Number of elements (length)	length(x)
First element	x[[1]]
_i_th element	x[[i]]
Last element	tail(x, 1)
First element with name g, as literal	x$g
First element with name g in variable	x[g]
Index of element e	which(x %in% e)	Use list(e) for non-scalar elements. Returns empty vector if missing.

Derive

Grow

Action	Code	Details
Append NULL	append(x, list(NULL))
Append scalar element e	append(x, e)	Not applicable for non-scalar vectors (length 0 or > 1)
Insert scalar element e at the _i_th position	append(x, e, after = i)
Replicate list n times	rep(x, n)
Replicate list to ensure length n	rep_len(x, n)	Cycles through elements until the length is reached

Shrink

Action	Code	Details
Slice from _a_th to _b_th element	x[a:b]
First n elements (head)	head(x, n)
First n elements (head)	x[1:n]
Last n elements (tail)	tail(x, n)
Drop first n elements	tail(x, -n)
Drop first n elements	x[-1:-n]
Drop last n elements	head(x, -n)
List of elements from a vector of indices vi	x[vi]	x[c(2, 3)]
List of elements from a vector of names vn	x[vn]	`x[c('a', 'b')]
Filter on condition or predicate function	Filter(function(e) e > 2, x)

Combine lists

Action	Code	Details
Concatenate lists	c(x, y, z)

Convert

Action	Code	Details
To vector (flatten)	unlist(x)	Warning: results strongly depend on element types: Elements are converted to common type. Vector elements are concatenated. Zero-length elements (e.g., NULL or integer()) are excluded.
To vector (flatten), drop names	unlist(x, use.names = FALSE)