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# std::merge

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt1, class InputIt2, class OutputIt > OutputIt merge( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, OutputIt d_first ); | (1) |
template< class InputIt1, class InputIt2, class OutputIt, class Compare> OutputIt merge( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, OutputIt d_first, Compare comp ); | (2) |

Merges two sorted ranges `[first1, last1)` and `[first2, last2)` into one sorted range beginning at `d_first`. The first version uses operator< to compare the elements, the second version uses the given comparison function `comp`. The relative order of equivalent elements is preserved.

The behavior is undefined if the destination range overlaps either of the input ranges (the input ranges may overlap each other).

### [edit] Parameters

first1, last1 | - | the first range of elements to merge
first2, last2 | - | the second range of elements to merge
d_first | - | the beginning of the destination range
comp | - | comparison function object (i.e. an object that satisfies the requirements of `Compare`) which returns ​true if the first argument is less than (i.e. is ordered before) the second. The signature of the comparison function should be equivalent to the following: bool cmp(const Type1 &a, const Type2 &b); The signature does not need to have const &, but the function object must not modify the objects passed to it. The types Type1 and Type2 must be such that objects of types InputIt1 and InputIt2 can be dereferenced and then implicitly converted to both Type1 and Type2. ​
Type requirements
- `InputIt1` must meet the requirements of `InputIterator`.
- `InputIt2` must meet the requirements of `InputIterator`.
- `OutputIt` must meet the requirements of `OutputIterator`.

### [edit] Return value

An output iterator to element past the last element copied.

### [edit] Complexity

At most std::distance(first1, last1) + std::distance(first2, last2) - 1 comparisons.

### [edit] Notes

This algorithm performs a similar task as std::set_union does. Both consume two sorted input ranges and produce a sorted output with elements from both inputs. The difference bewteen these two algorithms is with handling values from both input ranges which compare equivalent (see notes on `LessThanComparable`). If any equivalent values appeared `n` times in the first range and `m` times in the second, `std::merge` would output all n+m occurrences whereas `std::set_union` would output std::max(n, m) ones only. So `std::merge` outputs exactly std::distance(first1, last1) + std::distance(first2, last2) values and `std::set_union` may produce less.

### [edit] Possible implementation

First version
    template<class InputIt1, class InputIt2, class OutputIt>
    OutputIt merge(InputIt1 first1, InputIt1 last1,
                   InputIt2 first2, InputIt2 last2,
                   OutputIt d_first)
    {
        for (; first1 != last1; ++d_first) {
            if (first2 == last2) {
                return std::copy(first1, last1, d_first);
            }
            if (*first2 < *first1) {
                *d_first = *first2;
                ++first2;
            } else {
                *d_first = *first1;
                ++first1;
            }
        }
        return std::copy(first2, last2, d_first);
    }
Second version
    template<class InputIt1, class InputIt2,
             class OutputIt, class Compare>
    OutputIt merge(InputIt1 first1, InputIt1 last1,
                   InputIt2 first2, InputIt2 last2,
                   OutputIt d_first, Compare comp)
    {
        for (; first1 != last1; ++d_first) {
            if (first2 == last2) {
                return std::copy(first1, last1, d_first);
            }
            if (comp(*first2, *first1)) {
                *d_first = *first2;
                ++first2;
            } else {
                *d_first = *first1;
                ++first1;
            }
        }
        return std::copy(first2, last2, d_first);
    }

### [edit] Example

Run this code

    #include <iostream>
    #include <iterator>
    #include <algorithm>
    #include <vector>
    #include <random>
    #include <functional>

    int main()
    {
        // fill the vectors with random numbers
        std::random_device rd;
        std::mt19937 mt(rd());
        std::uniform_int_distribution<> dis(0, 9);

        std::vector<int> v1(10), v2(10);
        std::generate(v1.begin(), v1.end(), std::bind(dis, std::ref(mt)));
        std::generate(v2.begin(), v2.end(), std::bind(dis, std::ref(mt)));

        // sort
        std::sort(v1.begin(), v1.end());
        std::sort(v2.begin(), v2.end());

        // output v1
        std::cout << "v1 : ";
        std::copy(v1.begin(), v1.end(), std::ostream_iterator<int>(std::cout, " "));
        std::cout << '\n';

        // output v2
        std::cout << "v2 : ";
        std::copy(v2.begin(), v2.end(), std::ostream_iterator<int>(std::cout, " "));
        std::cout << '\n';

        // merge
        std::vector<int> dst;
        std::merge(v1.begin(), v1.end(), v2.begin(), v2.end(), std::back_inserter(dst));

        // output
        std::cout << "dst: ";
        std::copy(dst.begin(), dst.end(), std::ostream_iterator<int>(std::cout, " "));
        std::cout << '\n';
    }

Possible output:

    v1 : 0 1 3 4 4 5 5 8 8 9
    v2 : 0 2 2 3 6 6 8 8 8 9
    dst: 0 0 1 2 2 3 3 4 4 5 5 6 6 8 8 8 8 8 9 9

### [edit] See also

inplace_merge | merges two ordered ranges in-place (function template) [edit]
set_union | computes the union of two sets (function template) [edit]
sort | sorts a range into ascending order (function template) [edit]
stable_sort | sorts a range of elements while preserving order between equal elements (function template) [edit]
std::experimental::parallel::merge (parallelism TS) | parallelized version of `std::merge` (function template) [edit]

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