std::merge
Defined in header
<algorithm>
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template< class InputIt1, class InputIt2, class OutputIt >
OutputIt merge( InputIt1 first1, InputIt1 last1, |
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template< class InputIt1, class InputIt2, class OutputIt, class Compare>
OutputIt merge( InputIt1 first1, InputIt1 last1, |
(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).
Contents |
[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. |
Type requirements | ||
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InputIt1 must meet the requirements of InputIterator .
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InputIt2 must meet the requirements of InputIterator .
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OutputIt must meet the requirements of OutputIterator .
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[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 |
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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
#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
merges two ordered ranges in-place (function template) |
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computes the union of two sets (function template) |
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sorts a range into ascending order (function template) |
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sorts a range of elements while preserving order between equal elements (function template) |
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(parallelism TS)
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parallelized version of std::merge (function template) |