std::merge

 

C++

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Regular expressions library (C++11)

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Algorithm library

Functions
Non-modifying sequence operations
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each countcount_if mismatch equal	findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) fill fill_n transform generate generate_n removeremove_if remove_copyremove_copy_if	replacereplace_if replace_copyreplace_copy_if swap swap_ranges iter_swap reverse reverse_copy rotate rotate_copy random_shuffleshuffle  (C++11) unique unique_copy
Partitioning operations
is_partitioned (C++11) partition partition_copy (C++11)	stable_partition partition_point (C++11)
Sorting operations (on sorted ranges)
is_sorted (C++11) is_sorted_until (C++11) sort	partial_sort partial_sort_copy stable_sort nth_element
Binary search operations (on sorted ranges)
lower_bound upper_bound	binary_search equal_range
Set operations (on sorted ranges)
merge inplace_merge includes	set_difference set_intersection set_symmetric_difference set_union
Heap operations
is_heap (C++11) is_heap_until (C++11) make_heap	push_heap pop_heap sort_heap
Minimum/maximum operations
max max_element min min_element minmax (C++11) minmax_element (C++11)	lexicographical_compare is_permutation (C++11) next_permutation prev_permutation
Numeric operations
iota (C++11) accumulate inner_product	adjacent_difference partial_sum
C library
qsort	bsearch

 

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).

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 Type1 and Type2 respectively. ​
Type requirements
- InputIt1 must meet the requirements of InputIterator.
- InputIt2 must meet the requirements of InputIterator.
- OutputIt must meet the requirements of OutputIterator.

Return value

An output iterator to element past the last element copied.

Complexity

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

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); }

Example

Run this code

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

See also

inplace_merge	merges two ordered ranges in-place (function template)
sort	sorts a range into ascending order (function template)
stable_sort	sorts a range of elements while preserving order between equal elements (function template)