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inplace_merge

inplace_merge PrototypeInplace_merge is an overloaded name: there are actually two inplace_merge functions.

template <class BidirectionalIterator>
inline void inplace_merge(BidirectionalIterator first,
                          BidirectionalIterator middle,
                          BidirectionalIterator last);

template <class BidirectionalIterator, class StrictWeakOrdering>
inline void inplace_merge(BidirectionalIterator first,
                          BidirectionalIterator middle,
                          BidirectionalIterator last, StrictWeakOrdering comp);

DescriptionInplace_merge combines two consecutive sorted ranges [first, middle) and [middle, last) into a single sorted range [first, last). That is, it starts with a range [first, last) that consists of two pieces each of which is in ascending order, and rearranges it so that the entire range is in ascending order. Inplace_merge is stable, meaning both that the relative order of elements within each input range is preserved, and that for equivalent [1] elements in both input ranges the element from the first range precedes the element from the second.

The two versions of inplace_merge differ in how elements are compared. The first version uses operator<. That is, the input ranges and the output range satisfy the condition that for every pair of iterators i and j such that i precedes j, *j < *i is false. The second version uses the function object comp. That is, the input ranges and the output range satisfy the condition that for every pair of iterators i and j such that i precedes j, comp(*j, *i) is false.

Definition Defined in algo.h. Requirements on types For the first version:

BidirectionalIterator is a model of Bidirectional Iterator.
BidirectionalIterator is mutable.
BidirectionalIterator's value type is a model of LessThan Comparable.
The ordering on objects of BidirectionalIterator's value type is a strict weak ordering, as defined in the LessThan Comparable requirements.

For the second version:

BidirectionalIterator is a model of Bidirectional Iterator.
BidirectionalIterator is mutable.
StrictWeakOrdering is a model of Strict Weak Ordering.
BidirectionalIterator's value type is convertible to StrictWeakOrdering's argument type.

Preconditions For the first version:

[first, middle) is a valid range.
[middle, last) is a valid range.
[first, middle) is in ascending order. That is, for every pair of iterators i and j in [first, middle) such that i precedes j, *j < *i is false.
[middle, last) is in ascending order. That is, for every pair of iterators i and j in [middle, last) such that i precedes j, *j < *i is false.

For the second version:

[first, middle) is a valid range.
[middle, last) is a valid range.
[first, middle) is in ascending order. That is, for every pair of iterators i and j in [first, middle) such that i precedes j, comp(*j, *i) is false.
[middle, last) is in ascending order. That is, for every pair of iterators i and j in [middle, last) such that i precedes j, comp(*j, *i) is false.

ComplexityInplace_merge is an adaptive algorithm: it attempts to allocate a temporary memory buffer, and its run-time complexity depends on how much memory is available. Inplace_merge performs no comparisons if [first, last) is an empty range. Otherwise, worst-case behavior (if no auxiliary memory is available) is O(N log(N)), where N is last - first, and best case (if a large enough auxiliary memory buffer is available) is at most (last - first) - 1 comparisons. Example

int main()
{
  int A[] = { 1, 3, 5, 7, 2, 4, 6, 8 };

  inplace_merge(A, A + 4, A + 8);
  copy(A, A + 8, ostream_iterator<int>(cout, " "));  
  // The output is "1 2 3 4 5 6 7 8".
}

Notes

[1] Note that you may use an ordering that is a strict weak ordering but not a total ordering; that is, there might be values x and y such that x < y, x > y, and x == y are all false. (See the LessThan Comparable requirements for a fuller discussion.) Two elements x and y are equivalent if neither x < y nor y < x. If you're using a total ordering, however (if you're using strcmp, for example, or if you're using ordinary arithmetic comparison on integers), then you can ignore this technical distinction: for a total ordering, equality and equivalence are the same.

See alsomerge, set_union, sort STL Main Page

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