unique {base}  R Documentation 
unique
returns a vector, data frame or array like x
but with duplicate elements/rows removed.
unique(x, incomparables = FALSE, ...)
## Default S3 method:
unique(x, incomparables = FALSE, fromLast = FALSE,
nmax = NA, ...)
## S3 method for class 'matrix'
unique(x, incomparables = FALSE, MARGIN = 1,
fromLast = FALSE, ...)
## S3 method for class 'array'
unique(x, incomparables = FALSE, MARGIN = 1,
fromLast = FALSE, ...)
x 
a vector or a data frame or an array or 
incomparables 
a vector of values that cannot be compared.

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 
nmax 
the maximum number of unique items expected (greater than one).
See 
... 
arguments for particular methods. 
MARGIN 
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
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 ("NA"
) are regarded as equal, numeric and
complex ones differing from NaN
; character strings will be compared in a
“common encoding”; for details, see match
(and
duplicated
) which use the same concept.
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!
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
.
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
elements.
rle
which is the equivalent of the Unix uniq c
command.
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)