duplicated {base}

R Documentation

Determine Duplicate Elements

Description

Determines which elements of a vector or data frame are duplicates of elements with smaller subscripts, and returns a logical vector indicating which elements (rows) are duplicates.

Usage

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

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

## S3 method for class 'array'
duplicated(x, incomparables = FALSE, MARGIN = 1L,
           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 reverse side, i.e., the last (or rightmost) of identical elements would correspond to duplicated=FALSE.
...	arguments for particular methods.
MARGIN	the array margin to be held fixed: see apply.

Details

This is a generic function with methods for vectors (including lists), data frames and arrays (including matrices).

duplicated(x, fromLast=TRUE) is equivalent to but faster than rev(duplicated(rev(x))).

The data frame method works by pasting together a character representation of the rows separated by \r, so may be imperfect if the data frame has characters with embedded carriage returns or columns which do not reliably map to characters.

The array method calculates for each element of the sub-array specified by MARGIN if the remaining dimensions are identical to those for an earlier (or later, when fromLast=TRUE) element (in row-major order). This would most commonly be used to find duplicated rows (the default) or columns (with MARGIN = 2).

Missing values are regarded as equal, but NaN is not equal to NA_real_.

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.

Value

For a vector input, a logical vector of the same length as x. For a data frame, a logical vector with one element for each row. For a matrix or array, a logical array with the same dimensions and dimnames.

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

unique.

Examples

x <- c(9:20, 1:5, 3:7, 0:8)
## extract unique elements
(xu <- x[!duplicated(x)])
## similar, but not the same:
(xu2 <- x[!duplicated(x, fromLast = TRUE)])

## xu == unique(x) but unique(x) is more efficient
stopifnot(identical(xu,  unique(x)),
          identical(xu2, unique(x, fromLast = TRUE)))

duplicated(iris)[140:143]

duplicated(iris3, MARGIN = c(1, 3))

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