unique {base}R Documentation

Extract Unique Elements


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


unique(x, incomparables = FALSE, ...)

## S3 method for class 'matrix':
unique(x, incomparables = FALSE, MARGIN = 1, ...)

## S3 method for class 'array':
unique(x, incomparables = FALSE, MARGIN = 1, ...)


x a vector or a data frame or an array or NULL.
incomparables a vector of values that cannot be compared. Currently, FALSE is the only possible value, meaning that all values can be compared.
... 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 row-major order). This would most commonly be used for matrices to find unique rows (the default) or columns (with MARGIN = 2).


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.

See Also

duplicated which gives the indices of duplicated elements.


unique(c(3:5, 11:8, 8 + 0:5))
length(unique(sample(100, 100, replace=TRUE)))
## approximately 100(1 - 1/e) = 63.21


[Package base version 2.5.0 Index]