# unique_copy

Category: algorithms | Component type: function

### Prototype

`Unique_copy` is an overloaded name; there are actually two `unique_copy` functions.

    template <class InputIterator, class OutputIterator>
    OutputIterator unique_copy(InputIterator first, InputIterator last,
                               OutputIterator result);

    template <class InputIterator, class OutputIterator, class BinaryPredicate>
    OutputIterator unique_copy(InputIterator first, InputIterator last,
                               OutputIterator result,
                               BinaryPredicate binary_pred);

### Description

`Unique_copy` copies elements from the range `[first, last)` to a range beginning with `result`, except that in a consecutive group of duplicate elements only the first one is copied. The return value is the end of the range to which the elements are copied. This behavior is similar to the Unix filter `uniq`.

The reason there are two different versions of `unique_copy` is that there are two different definitions of what it means for a consecutive group of elements to be duplicates. In the first version, the test is simple equality: the elements in a range `[f, l)` are duplicates if, for every iterator `i` in the range, either `i == f` or else `*i == *(i-1)`. In the second, the test is an arbitrary Binary Predicate `binary_pred`: the elements in `[f, l)` are duplicates if, for every iterator `i` in the range, either `i == f` or else `binary_pred(*i, *(i-1))` is `true`. [1]

### Definition

Defined in the standard header algorithm, and in the nonstandard backward-compatibility header algo.h.

### Requirements on types

For the first version:

- `InputIterator` is a model of Input Iterator.
- `InputIterator`'s value type is Equality Comparable.
- `OutputIterator` is a model of Output Iterator.
- `InputIterator`'s value type is convertible to a type in `OutputIterator`'s set of value types.

For the second version:

- `InputIterator` is a model of Input Iterator.
- `BinaryPredicate` is a model of Binary Predicate. [2]
- `InputIterator`'s value type is convertible to first argument type and to `BinaryPredicate`'s second argument type.
- `OutputIterator` is a model of Output Iterator.
- `InputIterator`'s value type is convertible to a type in `OutputIterator`'s set of value types.

### Preconditions

- `[first, last)` is a valid range.
- There is enough space to hold all of the elements being copied. More formally, if there are `n` elements in the range `[first, last)` after duplicates are removed from consecutive groups, then `[result, result + n)` must be a valid range.

### Complexity

Linear. Exactly `last - first` applications of `operator==` (in the case of the first version of `unique`) or of `binary_pred` (in the case of the second version), and at most `last - first` assignments.

### Example

Print all of the numbers in an array, but only print the first one in a consecutive group of identical numbers.

    const int A[] = {2, 7, 7, 7, 1, 1, 8, 8, 8, 2, 8, 8};
    unique_copy(A, A + sizeof(A) / sizeof(int),
                ostream_iterator<int>(cout, " "));
    // The output is "2 7 1 8 2 8".

### Notes

[1] Strictly speaking, the first version of `unique_copy` is redundant: you can achieve the same functionality by using an object of class `equal_to` as the Binary Predicate argument. The first version is provided strictly for the sake of convenience: testing for equality is an important special case.

[2] `BinaryPredicate` is not required to be an equivalence relation. You should be cautious, though, about using `unique_copy` with a Binary Predicate that is not an equivalence relation: you could easily get unexpected results.

### See also

Binary Predicate, `unique`, `remove_copy`, `remove_copy_if`, `adjacent_find`

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