gather() takes a collection of elements defined by a pair of iterators and moves the ones satisfying a predicate to them to a position (called the pivot) within the sequence. The algorithm is stable. The result is a pair of iterators that contains the items that satisfy the predicate.
template <typename ForwardIterator, typename Pred> std::pair<ForwardIterator,ForwardIterator> gather ( ForwardIterator first, ForwardIterator last, ForwardIterator pivot, Pred pred );
Given an sequence containing: int [] arr = { 0 1 2 3 4 5 6 7 8 9 };
A call to gather ( arr, arr + 10, arr + 4, IsEven) will result in:
1 3 0 2 4 6 8 5 7 9 |---|-----| first | second pivot
where first and second are the fields of the pair returned by the call.
I don't understand why the matches are inserted between 3 and 5, rather than (say) 1 and 3. You need to specify exactly what the algorithm does (and does not) guarantee.
Notice the pivot is "arr + 4", which holds the same value (4) before and after the call. It seems to me that the algorithm operates independently on the sub-ranges [first, pivot) and [pivot + 1, last), for the first one moving elements satisfying the predicate to just before the pivot (while maintaining the relative order), and for the second one to just after the pivot. Am I understanding that correctly? Regards, Nate