Difference between revisions of "cpp/algorithm/ranges/remove"

Latest revision as of 21:29, 20 May 2024

Removes all elements satisfying specific criteria from the range [first, last) and returns a subrange [ret, last), where ret is a past-the-end iterator for the new end of the range.

Removing is done by shifting (by means of move assignment) the elements in the range in such a way that the elements that are not to be removed appear in the beginning of the range. Relative order of the elements that remain is preserved and the physical size of the container is unchanged. Iterators pointing to an element between the new logical end and the physical end of the range are still dereferenceable, but the elements themselves have unspecified values (as per MoveAssignable post-condition).

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

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

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

[edit] Parameters

[edit] Return value

{ret, last}, where [first, ret) is the resulting subrange after removal, and the elements in subrange [ret, last) are all in valid but unspecified state, i.e. [ret, last) is the subrange to be erased.

[edit] Complexity

Exactly N applications of the corresponding predicate and any projection, where N = ranges::distance(first, last), and N - 1 move operations at worst.

[edit] Notes

A call to ranges::remove is typically followed by a call to a container's erase member function, which erases the unspecified values and reduces the physical size of the container to match its new logical size. These two invocations together constitute a so-called Erase-remove idiom, which can be achieved by the free function std::erase that has overloads for all standard sequence containers, or std::erase_if that has overloads for all standard containers.

The similarly-named container member functions list::remove, list::remove_if, forward_list::remove, and forward_list::remove_if erase the removed elements.

These algorithms usually cannot be used with associative containers such as std::set and std::map because their iterator types do not dereference to MoveAssignable types (the keys in these containers are not modifiable).

Because ranges::remove takes value by reference, it can have unexpected behavior if it is a reference to an element of the range [first, last).

[edit] Possible implementation

struct remove_fn
{
    template<std::permutable I, std::sentinel_for<I> S, class Proj = std::identity,
             class T = std::projected_value_t<I, Proj>>
    requires std::indirect_binary_predicate
                 <ranges::equal_to, std::projected<I, Proj>, const T*>
    constexpr ranges::subrange<I>
        operator()(I first, S last, const T& value, Proj proj = {}) const
    {
        first = ranges::find(std::move(first), last, value, proj);
        if (first != last)
        {
            for (I i{std::next(first)}; i != last; ++i)
                if (value != std::invoke(proj, *i))
                {
                    *first = ranges::iter_move(i);
                    ++first;
                }
        }
        return {first, last};
    }
 
    template<ranges::forward_range R, class Proj = std::identity,
             class T = std::projected_value_t<ranges::iterator_t<R>, Proj>>
    requires std::permutable<ranges::iterator_t<R>> &&
             std::indirect_binary_predicate
                 <ranges::equal_to,
                  std::projected<ranges::iterator_t<R>, Proj>, const T*>
    constexpr ranges::borrowed_subrange_t<R>
        operator()(R&& r, const T& value, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), value, std::move(proj));
    }
};
 
inline constexpr remove_fn remove {};

struct remove_if_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(std::move(first), last, pred, proj);
        if (first != last)
        {
            for (I i{std::next(first)}; i != last; ++i)
                if (!std::invoke(pred, std::invoke(proj, *i)))
                {
                    *first = ranges::iter_move(i);
                    ++first;
                }
        }
        return {first, 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), pred, std::move(proj));
    }
};
 
inline constexpr remove_if_fn remove_if {};

[edit] Notes

[edit] Example

#include <algorithm>
#include <cassert>
#include <complex>
#include <cctype>
#include <iomanip>
#include <iostream>
#include <string>
#include <string_view>
#include <vector>
 
int main()
{
    std::string v1{"No - Diagnostic - Required"};
    std::cout << std::quoted(v1) << " (v1, size: " << v1.size() << ")\n";
    const auto ret = std::ranges::remove(v1, ' ');
    std::cout << std::quoted(v1) << " (v1 after `remove`, size: " << v1.size() << ")\n";
    std::cout << ' ' << std::string(std::distance(v1.begin(), ret.begin()), '^') << '\n';
    v1.erase(ret.begin(), ret.end());
    std::cout << std::quoted(v1) << " (v1 after `erase`, size: " << v1.size() << ")\n\n";
 
    // remove_if with custom unary predicate:
    auto rm = [](char c) { return !std::isupper(c); };
    std::string v2{"Substitution Failure Is Not An Error"};
    std::cout << std::quoted(v2) << " (v2, size: " << v2.size() << ")\n";
    const auto [first, last] = std::ranges::remove_if(v2, rm);
    std::cout << std::quoted(v2) << " (v2 after `remove_if`, size: " << v2.size() << ")\n";
    std::cout << ' ' << std::string(std::distance(v2.begin(), first), '^') << '\n';
    v2.erase(first, last);
    std::cout << std::quoted(v2) << " (v2 after `erase`, size: " << v2.size() << ")\n\n";
 
    // creating a view into a container that is modified by `remove_if`:
    for (std::string s : {"Small Object Optimization", "Non-Type Template Parameter"})
        std::cout << std::quoted(s) << " => "
            << std::string_view{begin(s), std::ranges::remove_if(s, rm).begin()} << '\n';
 
    std::vector<std::complex<double>> nums{{2, 2}, {1, 3}, {4, 8}};
    #ifdef __cpp_lib_algorithm_default_value_type
        auto e = std::ranges::remove(nums, {1, 3}); // T gets deduced
    #else
        auto e = std::ranges::remove(nums, std::complex<double>{1, 3});
    #endif
    nums.erase(e.begin(), e.end());
    assert((nums == std::vector<std::complex<double>>{{2, 2}, {4, 8}}));
}

"No _ Diagnostic _ Required" (v1, size: 26)
"No_Diagnostic_Requiredired" (v1 after `remove`, size: 26)
 ^^^^^^^^^^^^^^^^^^^^^^
"No_Diagnostic_Required" (v1 after `erase`, size: 22)
 
"Substitution Failure Is Not An Error" (v2, size: 36)
"SFINAEtution Failure Is Not An Error" (v2 after `remove_if`, size: 36)
 ^^^^^^
"SFINAE" (v2 after `erase`, size: 6)
 
"Small Object Optimization" => SOO
"Non-Type Template Parameter" => NTTP

[edit] See also