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[ base  (version  3.6.2  ) ](/packages/base/versions/3.6.2)

#  unique: Extract Unique Elements

##  Description

` unique ` returns a vector, data frame or array like ` x ` but with duplicate
elements/rows removed.

##  Usage

    
    
    unique(x, incomparables = FALSE, …)
    
    # S3 method for default
    unique(x, incomparables = FALSE, fromLast = FALSE,
            nmax = NA, …)
    
    # S3 method for matrix
    unique(x, incomparables = FALSE, MARGIN = 1,
           fromLast = FALSE, …)
    
    # S3 method for array
    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 as ` x ` .

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 ](/link/names?package=base&version=3.6.2) ` or ` [ dimnames
](/link/dimnames?package=base&version=3.6.2) ` .

nmax

the maximum number of unique items expected (greater than one). See ` [
duplicated ](/link/duplicated?package=base&version=3.6.2) ` .

…

arguments for particular methods.

MARGIN

the array margin to be held fixed: a single integer.

##  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 ](/link/vector?package=base&version=3.6.2) ` )
or differ only in their attributes. In the worst case it is \\(O(n^2)\\).

##  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 row-major 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 ](/link/rle?package=base&version=3.6.2) ` ).

Missing values ( ` " [ NA ](/link/NA?package=base&version=3.6.2) " ` ) are
regarded as equal, numeric and complex ones differing from ` NaN ` ; character
strings will be compared in a “common encoding”; for details, see ` [ match
](/link/match?package=base&version=3.6.2) ` (and ` [ duplicated
](/link/duplicated?package=base&version=3.6.2) ` ) 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 floating-point numbers!

##  References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) _Th