Extract Unique Elements
unique returns a vector, data frame or array like
but with duplicate elements/rows removed.
unique(x, incomparables = FALSE, ...)"unique"(x, incomparables = FALSE, fromLast = FALSE, nmax = NA, ...)"unique"(x, incomparables = FALSE, MARGIN = 1, fromLast = FALSE, ...)"unique"(x, incomparables = FALSE, MARGIN = 1, fromLast = FALSE, ...)
- a vector or a data frame or an array or
- a vector of values that cannot be compared.
FALSEis a special value, meaning that all values can be compared, and may be the only value accepted for methods other than the default. It will be coerced internally to the same type as
- logical indicating if duplication should be considered
from the last, i.e., the last (or rightmost) of identical elements will
be kept. This only matters for
- the maximum number of unique items expected (greater than one).
- arguments for particular methods.
- the array margin to be held fixed: a single integer.
This is a generic function with methods for vectors, data frames and arrays (including matrices).
The array method calculates for each element of the dimension
MARGIN if the remaining dimensions are identical
to those for an earlier element (in row-major order). This would most
commonly be used for matrices to find unique rows (the default) or columns
MARGIN = 2).
Note that unlike the Unix command
uniq this omits
duplicated and not just repeated elements/rows. That
is, an element is omitted if it is equal to any previous element and
not just if it is equal the immediately previous one. (For the
Missing values are regarded as equal, but
NaN is not equal to
NA_real_. Character strings are regarded as equal if they are
in different encodings but would agree when translated to UTF-8.
incomparables will never be marked as duplicated.
This is intended to be used for a fairly small set of values and will
not be efficient for a very large set.
When used on a data frame with more than one column, or an array or matrix when comparing dimensions of length greater than one, this tests for identity of character representations. This will catch people who unwisely rely on exact equality of floating-point numbers!
Character strings will be compared as byte sequences if any input is
For a vector, an object of the same type of
x, but with only one copy of each duplicated element. No attributes are copied (so the result has no names).For a data frame, a data frame is returned with the same columns but possibly fewer rows (and with row names from the first occurrences of the unique rows).A matrix or array is subsetted by
[, drop = FALSE], so dimensions and dimnames are copied appropriately, and the result always has the same number of dimensions as
Using this for lists is potentially slow, especially if the elements
are not atomic vectors (see
vector) or differ only
in their attributes. In the worst case it is $O(n^2)$.
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
duplicated which gives the indices of duplicated
rle which is the equivalent of the Unix
x <- c(3:5, 11:8, 8 + 0:5) (ux <- unique(x)) (u2 <- unique(x, fromLast = TRUE)) # different order stopifnot(identical(sort(ux), sort(u2))) length(unique(sample(100, 100, replace = TRUE))) ## approximately 100(1 - 1/e) = 63.21 unique(iris)