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# std::unique_copy

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt, class OutputIt > OutputIt unique_copy( InputIt first, InputIt last, OutputIt d_first ); | (1) |
template< class InputIt, class OutputIt, class BinaryPredicate > OutputIt unique_copy( InputIt first, InputIt last, OutputIt d_first, BinaryPredicate p ); | (2) |

Copies the elements from the range `[first, last)`, to another range beginning at `d_first` in such a way that there are no consecutive equal elements. Only the first element of each group of equal elements is copied. The first version uses `operator==` to compare the elements, the second version uses the given binary predicate `p`.

### [edit] Parameters

first, last | - | the range of elements to process
d_first | - | the beginning of the destination range
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
- `InputIt` must meet the requirements of `InputIterator`.
- `OutputIt` must meet the requirements of `OutputIterator`.
- The type of dereferenced `InputIt` must meet the requirements of `CopyAssignable`.
- The type of dereferenced `InputIt` must meet the requirements of `CopyConstructible`. if neither InputIt nor OutputIt satisfies `ForwardIterator`

### [edit] Return value

Output iterator to the element past the last written element

### [edit] Possible implementation

First version
    template<class ForwardIt, class OutputIt>
    ForwardIt unique_copy(ForwardIt first, ForwardIt last,
                          OutputIt d_first)
    {

        if (first == last)
            return d_first;

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

        if (first == last)
            return d_first;

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

### [edit] Example

The following program trims all multiple consecutive spaces in a const string and prints the result

Run this code

    #include <string>
    #include <iostream>
    #include <algorithm>
    #include <iterator>

    int main()
    {
        std::string s1 = "The      string    with many       spaces!";
        std::cout << "before: " << s1 << '\n';

        std::string s2;
        std::unique_copy(s1.begin(), s1.end(), std::back_inserter(s2),
                         [](char c1, char c2){ return c1 == ' ' && c2 == ' '; });

        std::cout << "after:  " << s2 << '\n';
    }

Output:

    before: The      string    with many       spaces!
    after:  The string with many spaces!

### [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 | removes consecutive duplicate elements in a range (function template) [edit]

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