std::ranges::is_heap() algorithm
- since C++20
- Simplified
- Detailed
// (1)
constexpr bool is_heap( I first, S last, Comp comp = {}, Proj proj = {} );
// (2)
constexpr bool is_heap( R&& r, Comp comp = {}, Proj proj = {} );
The type of arguments are generic and have the following constraints:
I
-std::random_access_iterator
S
-std::sentinel_for<I>
R
-std::ranges::random_access_range
Comp
:- (1) -
std::indirect_strict_weak_order<std::projected<I, Proj>>
- (2) -
std::indirect_strict_weak_order<std::projected<ranges::iterator_t<R>, Proj>>
- (1) -
Proj
- (none)
The Proj
and Comp
template arguments have the following default types: std::identity
, ranges::less
for all overloads.
// (1)
template<
std::random_access_iterator I,
std::sentinel_for<I> S,
class Proj = std::identity,
std::indirect_strict_weak_order<std::projected<I, Proj>> Comp = ranges::less
>
constexpr bool is_heap( I first, S last, Comp comp = {}, Proj proj = {} );
// (2)
template<
ranges::random_access_range R,
class Proj = std::identity,
std::indirect_strict_weak_order<std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less
>
constexpr bool is_heap( R&& r, Comp comp = {}, Proj proj = {} );
Checks if the elements in range [first
; last
) are a max heap.
-
(1) Elements are compared using the given binary comparison function
comp
and projection objectproj
. -
(2) Same as (1), but uses
r
as the source range, as if usingranges::begin(r)
asfirst
andranges::end(r)
aslast
.
The function-like entities described on this page are niebloids.
Parameters
first last | The range of elements to examine. |
r | The range of elements to examine. |
pred | Predicate to apply to the projected elements. |
proj | The projection to apply to the elements. |
Return value
true
if the range is max heap, false
otherwise.
Complexity
Linear in the distance between first
and last
.
Exceptions
(none)
Possible implementation
is_heap(1) and is_heap(2)
struct is_heap_fn
{
template<std::random_access_iterator I, std::sentinel_for<I> S,
class Proj = std::identity, std::indirect_strict_weak_order<
std::projected<I, Proj>> Comp = ranges::less>
constexpr bool operator()(I first, S last, Comp comp = {}, Proj proj = {}) const
{
return (last == ranges::is_heap_until(first, last,
std::move(comp), std::move(proj)));
}
template<ranges::random_access_range R, class Proj = std::identity,
std::indirect_strict_weak_order<std::projected<ranges::iterator_t<R>, Proj>>
Comp = ranges::less>
constexpr bool operator()(R&& r, Comp comp = {}, Proj proj = {}) const
{
return (*this)(ranges::begin(r), ranges::end(r),
std::move(comp), std::move(proj));
}
};
inline constexpr is_heap_fn is_heap {};
Notes
A max heap is a range of elements [f
; l
), arranged with respect to comparator comp
and projection proj
, that has the following properties:
- Given
N
asl - f
,p = f[(i - 1) / 2]
, andq = f[i]
, for all0 < i < N
, the expressionstd::invoke(comp, std::invoke(proj, p), std::invoke(proj, q))
evaluates tofalse
. - A new element can be added using
ranges::push_heap
, in O(log(N)) time. - The first element can be removed using
ranges::pop_heap
, in O(log(N)) time.
Examples
#include <algorithm>
#include <bit>
#include <cmath>
#include <iostream>
#include <vector>
void out(const auto& what, int n = 1) { while (n-- > 0) std::cout << what; }
void draw_heap(auto const& v);
int main()
{
std::vector<int> v {3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5, 8, 9, 7, 9, 3, 2, 3, 8};
out("initially, v:\n");
for (auto i : v) std::cout << i << ' ';
out('\n');
if (!std::ranges::is_heap(v))
{
out("making heap...\n");
std::ranges::make_heap(v);
}
out("after make_heap, v:\n");
for (auto t {1U}; auto i : v)
std::cout << i << (std::has_single_bit(++t) ? " │ " : " ");
out("\n" "corresponding binary tree is:\n");
draw_heap(v);
}
void draw_heap(auto const& v)
{
auto bails = [](int n, int w)
{
auto b = [](int w) { out("┌"), out("─", w), out("┴"), out("─", w), out("┐"); };
n /= 2;
if (!n)
return;
for (out(' ', w); n-- > 0; )
b(w), out(' ', w + w + 1);
out('\n');
};
auto data = [](int n, int w, auto& first, auto last)
{
for (out(' ', w); n-- > 0 && first != last; ++first)
out(*first), out(' ', w + w + 1);
out('\n');
};
auto tier = [&](int t, int m, auto& first, auto last)
{
const int n {1 << t};
const int w {(1 << (m - t - 1)) - 1};
bails(n, w), data(n, w, first, last);
};
const int m {static_cast<int>(std::ceil(std::log2(1 + v.size())))};
auto first {v.cbegin()};
for (int i {}; i != m; ++i)
tier(i, m, first, v.cend());
}
initially, v:
3 1 4 1 5 9 2 6 5 3 5 8 9 7 9 3 2 3 8
making heap...
after make_heap, v:
9 │ 8 9 │ 6 5 8 9 │ 3 5 3 5 3 4 7 2 │ 1 2 3 1
corresponding binary tree is:
9
┌───────┴───────┐
8 9
┌───┴───┐ ┌───┴───┐
6 5 8 9
┌─┴─┐ ┌─┴─┐ ┌─┴─┐ ┌─┴─┐
3 5 3 5 3 4 7 2
┌┴┐ ┌┴┐ ┌┴┐ ┌┴┐ ┌┴┐ ┌┴┐ ┌┴┐ ┌┴┐
1 2 3 1
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