std::nth_element - cppreference.com (original) (raw)

nth_element rearranges elements in [first, last) such that after the rearrangement:

• The element pointed at by nth is changed to whatever element would occur in that position if [first, last) were sorted.
• For every iterator i in [first, nth) and every iterator j in [nth, last), the following condition is met:

1,2) bool(*j < *i)(until C++20)std::less{}(*j, *i)(since C++20) is false.

3,4) bool(comp(*j, *i)) is false.

1. Elements are hypothetically sorted with respect to operator<(until C++20)std::less{}(since C++20).

2. Elements are hypothetically sorted with respect to comp.

2,4) Same as (1,3), but executed according to policy.

These overloads participate in overload resolution only if all following conditions are satisfied:

If any of the following conditions is satisfied, the behavior is undefined:

• [first, nth) or [nth, last) is not a valid range.

Contents

• 1 Parameters
• 2 Complexity
• 3 Exceptions
• 4 Possible implementation
• 5 Notes
• 6 Example
• 7 Defect reports
• 8 See also

[edit] Parameters

[edit] Complexity

Given \(\scriptsize N\)N as last - first:

1. \(\scriptsize O(N)\)O(N) comparisons using operator<(until C++20)std::less{}(since C++20) on average.

2. \(\scriptsize O(N)\)O(N) comparisons using operator<(until C++20)std::less{}(since C++20), and \(\scriptsize O(N \cdot \log(N))\)O(N·log(N)) swaps.

3. \(\scriptsize O(N)\)O(N) applications of the comparator comp on average.

4. \(\scriptsize O(N)\)O(N) applications of the comparator comp, and \(\scriptsize O(N \cdot \log(N))\)O(N·log(N)) swaps.

[edit] Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

[edit] Possible implementation

See also the implementations in libstdc++, libc++, and MSVC STL.

[edit] Notes

The algorithm used is typically Introselect although other Selection algorithm with suitable average-case complexity are allowed.

[edit] Example

#include #include #include #include #include #include   void printVec(const std::vector& vec) { std::cout << "v = {"; for (char sep[]{0, ' ', 0}; const int i : vec) std::cout << sep << i, sep[0] = ','; std::cout << "};\n"; }   int main() { std::vector v{5, 10, 6, 4, 3, 2, 6, 7, 9, 3}; printVec(v);   auto m = v.begin() + v.size() / 2; std::nth_element(v.begin(), m, v.end()); std::cout << "\nThe median is " << v[v.size() / 2] << '\n'; // The consequence of the inequality of elements before/after the Nth one: assert(std::accumulate(v.begin(), m, 0) < std::accumulate(m, v.end(), 0)); printVec(v);   // Note: comp function changed std::nth_element(v.begin(), v.begin() + 1, v.end(), std::greater{}); std::cout << "\nThe second largest element is " << v[1] << '\n'; std::cout << "The largest element is " << v[0] << '\n'; printVec(v); }

Possible output:

v = {5, 10, 6, 4, 3, 2, 6, 7, 9, 3};   The median is 6 v = {3, 2, 3, 4, 5, 6, 10, 7, 9, 6};   The second largest element is 9 The largest element is 10 v = {10, 9, 6, 7, 6, 3, 5, 4, 3, 2};

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also