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Showing content from https://cplusplus.com/merge below:

function template

<algorithm>

std::merge default (1) 
template <class InputIterator1, class InputIterator2, class OutputIterator>  OutputIterator merge (InputIterator1 first1, InputIterator1 last1,                        InputIterator2 first2, InputIterator2 last2,                        OutputIterator result);
custom (2) 
template <class InputIterator1, class InputIterator2,          class OutputIterator, class Compare>  OutputIterator merge (InputIterator1 first1, InputIterator1 last1,                        InputIterator2 first2, InputIterator2 last2,                        OutputIterator result, Compare comp);

Merge sorted ranges

Combines the elements in the sorted ranges [first1,last1) and [first2,last2), into a new range beginning at result with all its elements sorted.

The elements are compared using operator< for the first version, and comp for the second. The elements in both ranges shall already be ordered according to this same criterion (operator< or comp). The resulting range is also sorted according to this.

The behavior of this function template is equivalent to:


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template <class InputIterator1, class InputIterator2, class OutputIterator>
  OutputIterator merge (InputIterator1 first1, InputIterator1 last1,
                        InputIterator2 first2, InputIterator2 last2,
                        OutputIterator result)
{
  while (true) {
    if (first1==last1) return std::copy(first2,last2,result);
    if (first2==last2) return std::copy(first1,last1,result);
    *result++ = (*first2<*first1)? *first2++ : *first1++;
  }
}

Parameters
first1, last1
Input iterators to the initial and final positions of the first sorted sequence. The range used is [first1,last1), which contains all the elements between first1 and last1, including the element pointed by first1 but not the element pointed by last1.
first2, last2
Input iterators to the initial and final positions of the second sorted sequence. The range used is [first2,last2).
result
Output iterator to the initial position of the range where the resulting combined range is stored. Its size is equal to the sum of both ranges above.
comp
Binary function that accepts two arguments of the types pointed by the iterators, and returns a value convertible to bool. The value returned indicates whether the first argument is considered to go before the second in the specific strict weak ordering it defines.
The function shall not modify any of its arguments.
This can either be a function pointer or a function object.

The ranges shall not overlap.
The elements in both input ranges should be assignable to the elements in the range pointed by result. They should also be comparable (with operator< for (1), and with comp for (2)).

Return value An iterator pointing to the past-the-end element in the resulting sequence.

Example 
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// merge algorithm example
#include <iostream>     // std::cout
#include <algorithm>    // std::merge, std::sort
#include <vector>       // std::vector

int main () {
  int first[] = {5,10,15,20,25};
  int second[] = {50,40,30,20,10};
  std::vector<int> v(10);

  std::sort (first,first+5);
  std::sort (second,second+5);
  std::merge (first,first+5,second,second+5,v.begin());

  std::cout << "The resulting vector contains:";
  for (std::vector<int>::iterator it=v.begin(); it!=v.end(); ++it)
    std::cout << ' ' << *it;
  std::cout << '\n';

  return 0;
}

Output: 
The resulting vector contains: 5 10 10 15 20 20 25 30 40 50


Complexity Up to linear in (1+count1-count2), where countX is the distance between firstX and lastX: Compares and assigns all elements.

Data races The objects in the ranges [first1,last1) and [first2,last2)are accessed.
The objects in the range between result and the returned value are modified.

Exceptions Throws if any of element comparisons, the element assignments or the operations on iterators throws.
Note that invalid arguments cause undefined behavior.

See also
inplace_merge
Merge consecutive sorted ranges (function template)
set_union
Union of two sorted ranges (function template)
copy
Copy range of elements (function template)

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