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# std::unique

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class ForwardIt > ForwardIt unique( ForwardIt first, ForwardIt last ); | (1) |
template< class ForwardIt, class BinaryPredicate > ForwardIt unique( ForwardIt first, ForwardIt last, BinaryPredicate p ); | (2) |

Removes all consecutive duplicate elements from the range `[first, last)` and returns a past-the-end iterator for the new logical end of the range. The first version uses `operator==` to compare the elements, the second version uses the given binary predicate `p`.

Removing is done by shifting the elements in the range in such a way that elements to be erased are overwritten. Relative order of the elements that remain is preserved and the physical size of the container is unchanged. Iterators pointing to an element between the new logical end and the physical end of the range are still dereferenceable, but the elements themselves have unspecified values. A call to `unique` is typically followed by a call to a container's `erase` method, which erases the unspecified values and reduces the physical size of the container to match its new logical size.

### [edit] Parameters

first, last | - | the range of elements to process
p | - | binary predicate which returns ​true if the elements should be treated as equal. The signature of the predicate function should be equivalent to the following: bool pred(const Type1 &a, const Type2 &b); The signature does not need to have const &, but the function must not modify the objects passed to it. The types Type1 and Type2 must be such that an object of type ForwardIt can be dereferenced and then implicitly converted to both of them. ​
Type requirements
- `ForwardIt` must meet the requirements of `ForwardIterator`.
- The type of dereferenced `ForwardIt` must meet the requirements of `MoveAssignable`.

### [edit] Return value

Forward iterator to the new end of the range

### [edit] Possible implementation

First version
    template<class ForwardIt>
    ForwardIt unique(ForwardIt first, ForwardIt last)
    {
        if (first == last)
            return last;

        ForwardIt result = first;
        while (++first != last) {
            if (!(*result == *first)) {
                *(++result) = *first;
            }
        }
        return ++result;
    }
Second version
    template<class ForwardIt, class BinaryPredicate>
    ForwardIt unique(ForwardIt first, ForwardIt last,
                           BinaryPredicate p)
    {
        if (first == last)
            return last;

        ForwardIt result = first;
        while (++first != last) {
            if (!p(*result, *first)) {
                *(++result) = *first;
            }
        }
        return ++result;
    }

### [edit] Example

The following code removes all consecutive equivalent elements from a vector of integers.

Run this code

    #include <iostream>
    #include <algorithm>
    #include <vector>

    int main()
    {
        std::vector<int> v{1, 2, 2, 2, 3, 3, 2, 2, 1};
        std::vector<int>::iterator last;

        last = std::unique(v.begin(), v.end()); // 1 2 3 2 1 3 2 2 1
                                                //           ^
        for (std::vector<int>::iterator it = v.begin(); it != last; ++it) {
            std::cout << *it << " ";
        }
        std::cout << "\n";
    }

Output:

    1 2 3 2 1

### [edit] Complexity

linear in the distance between `first` and `last`

### [edit] See also

adjacent_find | finds two identical (or some other relationship) items adjacent to each other (function template) [edit]
unique_copy | creates a copy of some range of elements that contains no consecutive duplicates (function template) [edit]
remove remove_if | removes elements satisfying specific criteria (function template) [edit]

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