std::unique

 

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all_of any_of none_of (C++11) (C++11) (C++11) for_each count count_if mismatch equal	find find_if find_if_not (C++11) find_end find_first_of adjacent_find search search_n
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copy copy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) fill fill_n transform generate generate_n remove remove_if remove_copy remove_copy_if	replace replace_if replace_copy replace_copy_if swap swap_ranges iter_swap reverse reverse_copy rotate rotate_copy random_shuffle shuffle   (C++11) unique unique_copy
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is_partitioned (C++11) partition partition_copy (C++11)	stable_partition partition_point (C++11)
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C library
qsort	bsearch

 

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

Removes all consecutive duplicate elements from the range [first, last). Removing is done by shifting the range when needed in such a way that elements to be erased are overwritten. Only the first element in each group of equal elements is left. The elements between the old and the new end or the range are left intact. The first version uses operator== to compare the elements, the second version uses the given binary predicate p.

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 ForwardIterator can be dereferenced and then implicitly converted to both of them. ​

Return value

forward iterator to the new end of the range

Possible implementation

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

Example

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

#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

Complexity

linear in the distance between first and last

See also

adjacent_find	finds two identical (or some other relationship) items adjacent to each other (function template)
unique_copy	creates a copy of some range of elements that contains no consecutive duplicates (function template)