unique
Extract Unique Elements
unique
returns a vector, data frame or array like x
but with duplicate elements/rows removed.
Usage
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, ...)
Arguments
 x
 a vector or a data frame or an array or
NULL
.  incomparables
 a vector of values that cannot be compared.
FALSE
is 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 asx
.  fromLast
 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
names
ordimnames
.  nmax
 the maximum number of unique items expected (greater than one).
See
duplicated
.  ...
 arguments for particular methods.
 MARGIN
 the array margin to be held fixed: a single integer.
Details
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
specified by MARGIN
if the remaining dimensions are identical
to those for an earlier element (in rowmajor order). This would most
commonly be used for matrices to find unique rows (the default) or columns
(with 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
latter, see rle
).
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 UTF8.
Values in 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 floatingpoint numbers!
Character strings will be compared as byte sequences if any input is
marked as "bytes"
(see Encoding
).
Value

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 x
.
Warning
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 $O(n^2)$.
References
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
See Also
duplicated
which gives the indices of duplicated
elements.
rle
which is the equivalent of the Unix uniq c
command.
Examples
library(base)
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)