R: Extract Unique Elements

unique {base} | R Documentation

## 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, ...)

    ## Default S3 method:
    unique(x, incomparables = FALSE, fromLast = FALSE, ...)

    ## 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, ...)

### Arguments

`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.
`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` or `dimnames`.
`...` | 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 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 identical to any previous element and not just if it is the same as the immediately previous one.

### 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

    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)

[Package base version 2.6.2 Index]