std::ranges::partition

Defined in header `<algorithm>`
Call signature
template< std::permutable I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred > constexpr ranges::subrange<I> partition( I first, S last, Pred pred, Proj proj = {} );	(1)	(since C++20)
template< ranges::forward_range R, class Proj = std::identity, std::indirect_unary_predicate< std::projected<ranges::iterator_t<R>, Proj>> Pred > requires std::permutable<ranges::iterator_t<R>> constexpr ranges::borrowed_subrange_t<R> partition( R&& r, Pred pred, Proj proj = {} );	(2)	(since C++20)

1) Reorders the elements in the range [first, last) in such a way that the projection proj of all elements for which the predicate pred returns true precede the projection proj of elements for which predicate pred returns false. Relative order of elements is not preserved.

2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are niebloids, that is:

Explicit template argument lists may not be specified when calling any of them.
None of them is visible to argument-dependent lookup.
When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.

In practice, they may be implemented as function objects, or with special compiler extensions.

Parameters

first, last	-	the range of elements to reorder
r	-	the range of elements to reorder
pred	-	predicate to apply to the projected elements
proj	-	projection to apply to the elements.

Return value

A subrange starting with an iterator to the first element of the second group and finishing with an iterator equal to last. (2) returns std::ranges::dangling if r is an rvalue of non-borrowed_range type.

Complexity

Given N = ranges::distance(first,last), exactly $N$ applications of the predicate and projection. At most $N/2$ swaps if I models ranges::bidirectional_iterator, and at most $N$ swaps otherwise.

Possible implementation

struct partition_fn {
  template<std::permutable I, std::sentinel_for<I> S, class Proj = std::identity,
           std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
  constexpr ranges::subrange<I>
  operator()(I first, S last, Pred pred, Proj proj = {}) const
  {
      first = ranges::find_if_not(first, last, std::ref(pred), std::ref(proj));
      if (first == last) {
          return {first, first};
      }
 
      auto begin = first;
      for (auto i = ranges::next(first); i != last; ++i) {
          if (std::invoke(pred, std::invoke(proj, *i))) {
              ranges::iter_swap(i, first);
              ++first;
          }
      }
      return {std::move(first), std::move(last)};
  }
 
  template<ranges::forward_range R, class Proj = std::identity,
           std::indirect_unary_predicate<
               std::projected<ranges::iterator_t<R>, Proj>> Pred>
  requires std::permutable<ranges::iterator_t<R>>
  constexpr ranges::borrowed_subrange_t<R>
  operator()(R&& r, Pred pred, Proj proj = {}) const
  {
    return (*this)(ranges::begin(r), ranges::end(r),
                   std::ref(pred), std::ref(proj));
  }
};
 
inline constexpr partition_fn partition;

Example

Run this code

#include <algorithm>
#include <forward_list>
#include <functional>
#include <iostream>
#include <iterator>
#include <ranges>
#include <vector>
 
namespace ranges = std::ranges;
 
template <class I, std::sentinel_for<I> S, class Cmp = ranges::less>
requires std::sortable<I, Cmp>
void quicksort(I first, S last, Cmp cmp = Cmp{})
{
    using reference = std::iter_reference_t<I>;
 
    if (first == last)
        return;
 
    auto size = ranges::distance(first, last);
    auto pivot = ranges::next(first, size - 1);
    ranges::iter_swap(pivot, ranges::next(first, size / 2));
 
    ranges::subrange tail = ranges::partition(first, pivot, [=](reference em) {
        // em < pivot
        return std::invoke(cmp, em, *pivot);
    });
 
    ranges::iter_swap(pivot, tail.begin());
    quicksort(first, tail.begin(), std::ref(cmp));
    quicksort(ranges::next(tail.begin()), last, std::ref(cmp));
}
 
int main()
{
    std::ostream_iterator<int> cout {std::cout, " "};
 
    std::vector<int> v {0,1,2,3,4,5,6,7,8,9};
    std::cout << "Original vector:  \t";
    ranges::copy(v, cout);
 
    auto tail = ranges::partition(v, [](int i){return i % 2 == 0;});
 
    std::cout << "\nPartitioned vector: \t";
    ranges::copy(ranges::begin(v), ranges::begin(tail), cout);
    std::cout << "│ ";
    ranges::copy(tail, cout);
 
    std::forward_list<int> fl {1, 30, -4, 3, 5, -4, 1, 6, -8, 2, -5, 64, 1, 92};
    std::cout << "\nUnsorted list: \t\t";
    ranges::copy(fl, cout);
 
    quicksort(ranges::begin(fl), ranges::end(fl), ranges::greater{});
    std::cout << "\nQuick-sorted list: \t";
    ranges::copy(fl, cout);
 
    std::cout << '\n';
}

Possible output:

Original vector:        0 1 2 3 4 5 6 7 8 9 
Partitioned vector:     0 8 2 6 4 │ 5 3 7 1 9 
Unsorted list:          1 30 -4 3 5 -4 1 6 -8 2 -5 64 1 92 
Quick-sorted list:      92 64 30 6 5 3 2 1 1 1 -4 -4 -5 -8

Compiler support
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Language
Standard library headers
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Feature test macros (C++20)
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Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
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Ranges library (C++20)
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Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

ranges::partition_copy (C++20)	copies a range dividing the elements into two groups (niebloid)
ranges::is_partitioned (C++20)	determines if the range is partitioned by the given predicate (niebloid)
ranges::stable_partition (C++20)	divides elements into two groups while preserving their relative order (niebloid)
partition	divides a range of elements into two groups (function template)