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,
            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, ...)

### 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` or `dimnames`.
`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 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`).

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 floating-point numbers!

### 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 4.0.5 Index]