ranges_count.md

count

• algorithm[meta header]
• std::ranges[meta namespace]
• function template[meta id-type]
• cpp20[meta cpp]

namespace std::ranges {
  template<input_iterator I, sentinel_for<I> S, class T, class Proj = identity>
    requires indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T*>
  constexpr iter_difference_t<I> count(I first, S last, const T& value, Proj proj = {}); // (1)

  template<input_range R, class T, class Proj = identity>
    requires indirect_binary_predicate<ranges::equal_to, projected<iterator_t<R>, Proj>, const T*>
  constexpr range_difference_t<R> count(R&& r, const T& value, Proj proj = {});          // (2)
}

• input_iterator[link /reference/iterator/input_iterator.md]
• input_range[link /reference/ranges/input_range.md]
• sentinel_for[link /reference/iterator/sentinel_for.md]
• identity[link /reference/functional/identity.md]
• iterator_t[link /reference/ranges/iterator_t.md]
• indirect_binary_predicate[link /reference/iterator/indirect_binary_predicate.md]
• ranges::equal_to[link /reference/functional/ranges_equal_to.md]
• projected[link /reference/iterator/projected.md]
• iter_difference_t[link /reference/iterator/iter_difference_t.md]
• range_difference_t[link /reference/ranges/range_difference_t.md]

概要

指定された値と等値な要素の数を数える。

• (1): イテレータペアで範囲を指定する
• (2): 範囲を直接指定する

テンプレートパラメータ制約

• (1):
  • Iがinput_iteratorである
  • SがIに対する番兵である
  • IをProjで射影した値と指定された値がequal_toによって等値比較できる
• (2):
  • Rがinput_rangeである
  • PredはRのイテレータをProjで射影した値がequal_toによって等値比較できる

戻り値

[first,last) 内のイテレータ i について、invoke(proj, *i) == value であるイテレータの数を返す

計算量

正確に last - first 回の比較を行う

例

#include <algorithm>
#include <iostream>
#include <array>

int main() {
  constexpr std::array v = { 1,4,3,3,1,2,2,1 };

  // 値が 1 の要素がいくつあるかを数える
  std::cout << "count of 1: " << std::ranges::count(v, 1) << std::endl;
}

• std::ranges::count[color ff0000]

出力

count of 1: 3

実装例

struct count_impl {
  template<input_iterator I, sentinel_for<I> S, class T, class Proj = identity>
    requires indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T*>
  constexpr iter_difference_t<I> operator()(I first, S last, const T& value, Proj proj = {}) const {
    iter_difference_t<I> count = 0;
    for ( ; first != last; ++first)
      if (value == invoke(proj, *first)) count++;
    return count;
  }

  template<input_range R, class T, class Proj = identity>
    requires indirect_binary_predicate<ranges::equal_to, projected<iterator_t<R>, Proj>, const T*>
  constexpr range_difference_t<R> operator()(R&& r, const T& value, Proj proj = {}) const {
    return (*this)(begin(r), end(r), value, ref(proj));
  }
};

inline constexpr count_impl count;

• input_iterator[link /reference/iterator/input_iterator.md]
• input_range[link /reference/ranges/input_range.md]
• sentinel_for[link /reference/iterator/sentinel_for.md]
• identity[link /reference/functional/identity.md]
• iterator_t[link /reference/ranges/iterator_t.md]
• indirect_binary_predicate[link /reference/iterator/indirect_binary_predicate.md]
• ranges::equal_to[link /reference/functional/ranges_equal_to.md]
• projected[link /reference/iterator/projected.md]
• iter_difference_t[link /reference/iterator/iter_difference_t.md]
• range_difference_t[link /reference/ranges/range_difference_t.md]
• invoke[link /reference/functional/invoke.md]
• begin[link /reference/ranges/begin.md]
• end[link /reference/ranges/end.md]
• ref[link /reference/functional/ref.md]

バージョン

言語

• C++20

処理系

• Clang: ??
• GCC: 10.1.0
• ICC: ??
• Visual C++: 2019 Update 10

参照

• N4861 25 Algorithms library