R: Determine Duplicate Elements

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 = 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 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`).

### 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))

[Package base version 2.6.2 Index]