# inplace_merge

Category: algorithms | Component type: function

### Prototype

`Inplace_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);

### Description

`Inplace_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`.

### Complexity

`Inplace_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 also

`merge`, `set_union`, `sort`

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