# gather [Mutating Algorithms]

## Functions

template<typename Iter , typename Pred >
std::pair< Iter, Iter > gather (Iter first, Iter last, Iter pivot, Pred pred)
template<typename BidirectionalRange , typename Pred >
std::pair< typename
boost::range_iterator
< BidirectionalRange >::type,
typename boost::range_iterator
< BidirectionalRange >::type >
gather (BidirectionalRange &range, typename boost::range_iterator< BidirectionalRange >::type pivot, Pred pred)

## Detailed Description

Date:
January 2008

`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.

Given an sequence containing:

```    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
```

The problem is broken down into two basic steps, namely, moving the items before the pivot and then moving the items from the pivot to the end. These "moves" are done with calls to stable_partition.

Storage Requirements:

The algorithm uses stable_partition, which will attempt to allocate temporary memory, but will work in-situ if there is none available.

Time Complexity:

If there is sufficient memory available, the run time is linear in `N`. If there is not any memory available, then the run time is `O(N log N)`.

## Function Documentation

 std::pair adobe::gather ( Iter first, Iter last, Iter pivot, Pred pred )

iterator-based gather implementation

Definition at line 78 of file gather.hpp.

 std::pair< typename boost::range_iterator::type, typename boost::range_iterator::type> adobe::gather ( BidirectionalRange & range, typename boost::range_iterator< BidirectionalRange >::type pivot, Pred pred )

range-based gather implementation

Definition at line 100 of file gather.hpp.