unique


function template
<algorithm>
template <class ForwardIterator>
  ForwardIterator unique ( ForwardIterator first, ForwardIterator last );

template <class ForwardIterator, class BinaryPredicate>
  ForwardIterator unique ( ForwardIterator first, ForwardIterator last,
                           BinaryPredicate pred );

Remove consecutive duplicates in range

Removes the duplicate consecutive elements from the range [first,last). This is done by removing all the elements that compare equal to the element right preceding them (only the first element in each group of consecutive equal elements is kept).

The comparison between elements is performed by either applying the == comparison operator, or the template parameter comp (for the second version) between them.

The behavior of this function template is equivalent to:
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template <class ForwardIterator>
  ForwardIterator unique ( ForwardIterator first, ForwardIterator last )
{
  ForwardIterator result=first;
  while (++first != last)
  {
    if (!(*result == *first))  // or: if (!pred(*result,*first)) for the pred version
      *(++result)=*first;
  }
  return ++result;
}


Notice that this function does not alter the elements past the new end, which keep their old values and are still accessible.

Parameters

first, last
Forward iterators to the initial and final positions of the sequence. The range used is [first,last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.
pred
Binary predicate taking two elements as argument and returning the result of the comparison between them, with true (non-zero) meaning that they are to be considered equal, and false (zero) for not-equal. This can either be a pointer to a function or an object whose class overloads operator().


Return value

A forward iterator pointing to the new end of the sequence, which now does not include the elements that were consecutive duplicates.

Example

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// unique algorithm example
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;

bool myfunction (int i, int j) {
  return (i==j);
}

int main () {
  int myints[] = {10,20,20,20,30,30,20,20,10};    // 10 20 20 20 30 30 20 20 10
  vector<int> myvector (myints,myints+9);
  vector<int>::iterator it;

  // using default comparison:
  it = unique (myvector.begin(), myvector.end()); // 10 20 30 20 10 30 20 20 10
                                                  //                ^

  myvector.resize( it - myvector.begin() );       // 10 20 30 20 10

  // using predicate comparison:
  unique (myvector.begin(), myvector.end(), myfunction);   // (no changes)

  // print out content:
  cout << "myvector contains:";
  for (it=myvector.begin(); it!=myvector.end(); ++it)
    cout << " " << *it;

  cout << endl;

  return 0;
}


Output:
myvector contains: 10 20 30 20 10


Complexity

At most, performs as many assignments and comparisons or applications of pred as the number of elements in the range [first,last).

See also

unique_copy Copy range removing duplicates (function template)
adjacent_find Find equal adjacent elements in range (function template)
remove Remove value from range (function template)
remove_if Remove elements from range (function template)