std::count, std::count_if

 

C++

言語

ヘッダ

コンセプト

言語サポートライブラリ

診断ライブラリ

ユーティリティライブラリ

文字列ライブラリ

コンテナライブラリ

アルゴリズムライブラリ

イテレータライブラリ

数値演算ライブラリ

入出力ライブラリ

ローカライゼーションライブラリ

正規表現ライブラリ(C++11)

アトミック操作ライブラリ(C++11)

スレッドサポートライブラリ(C++11)

ファイルシステムライブラリ(C++17)

技術仕様書

 

アルゴリズムライブラリ

実行ポリシー (C++17)
is_execution_policy	execution::seq execution::par execution::par_unseq	execution::sequenced_policy execution::parallel_policy execution::parallel_unsequenced_policy
非変更シーケンス操作
all_of any_of none_of (C++11) (C++11) (C++11) for_each for_each_n (C++17)	count count_if mismatch equal adjacent_find	find find_if find_if_not (C++11) find_end find_first_of	search search_n lexicographical_compare lexicographical_compare_3way (C++20)
変更シーケンス操作
copy copy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	remove remove_if replace replace_if reverse rotate unique random_shuffle (C++17以前)	remove_copy remove_copy_if replace_copy replace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
未初期化記憶域の操作
uninitialized_copy uninitialized_move (C++17)	uninitialized_copy_n (C++11) uninitialized_move_n (C++17)	uninitialized_fill	uninitialized_fill_n
分割操作
is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
ソート操作
is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
バイナリサーチ操作
lower_bound	upper_bound	binary_search	equal_range
集合操作 (ソート済み範囲に対する)
merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
ヒープ操作
is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
最小/最大演算
max max_element	min min_element	minmax (C++11) minmax_element (C++11)	clamp (C++17) compare_3way (C++20)
順列
is_permutation (C++11)	next_permutation	prev_permutation
数値演算
iota (C++11) inner_product adjacent_difference	accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17) exclusive_scan (C++17)	transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
C のライブラリ
qsort	bsearch

 

ヘッダ <algorithm> で定義
template<class InputIt, class T > typename iterator_traits<InputIt>::difference_type     count( InputIt first, InputIt last, const T &value );	(1)
template<class InputIt, class UnaryPredicate > typename iterator_traits<InputIt>::difference_type     count_if( InputIt first, InputIt last, UnaryPredicate p );	(2)

[first, last)範囲内の、特定の条件を満たす要素数を返す.最初のバージョンはvalueに等しい要素をカウントし、2番目のバージョンは述語関数pの戻り値がtrueである要素をカウントする.

パラメータ

first, last	-	検討する要素の範囲 Original: the range of elements to examine The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.
value	-	検索する値 Original: the value to search for The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.
p	-	unary predicate which returns ​true 必要な要素のために Original: for the required elements The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. . The signature of the predicate function should be equivalent to the following:  bool pred(const Type &a); The signature does not need to have const&, but the function must not modify the objects passed to it. The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type. ​
型の要件
- InputIt は InputIterator の要件を満たさなければなりません。

値を返します

条件を満たす要素の数.

Original:

number of elements satisfying the condition.

The text has been machine-translated via Google Translate.
You can help to correct and verify the translation. Click here for instructions.

複雑性

まさにlast - first比較/述語のアプリケーション

Original:

exactly last - first comparisons / applications of the predicate

The text has been machine-translated via Google Translate.
You can help to correct and verify the translation. Click here for instructions.

可能な実装

First version
template<class InputIt, class T>typename iterator_traits<InputIt>::difference_type count(InputIt first, InputIt last, const T& value){typename iterator_traits<InputIt>::difference_type ret =0;for(; first != last;++first){if(*first == value){ ret++;}}return ret;}
Second version
template<class InputIt, class UnaryPredicate>typename iterator_traits<InputIt>::difference_type count_if(InputIt first, InputIt last, UnaryPredicate p){typename iterator_traits<InputIt>::difference_type ret =0;for(; first != last;++first){if(p(*first)){ ret++;}}return ret;}

例

次のコードは、どのように多くの整数countでは目標値に一致するかを判断するためにstd::vectorを使用しています.

Original:

The following code uses count to determine how many integers in a std::vector match a target value.

The text has been machine-translated via Google Translate.
You can help to correct and verify the translation. Click here for instructions.

#include <algorithm>#include <iostream>#include <vector>   int main(){int data[]={1, 2, 3, 4, 4, 3, 7, 8, 9, 10};std::vector<int> v(data, data+10);   int target1 =3;int target2 =5;int num_items1 = std::count(v.begin(), v.end(), target1);int num_items2 = std::count(v.begin(), v.end(), target2);   std::cout<<"number: "<< target1 <<" count: "<< num_items1 <<'\n';std::cout<<"number: "<< target2 <<" count: "<< num_items2 <<'\n';}

出力:

number: 3 count: 2 number: 5 count: 0

この例では、3で割り切れる要素をカウントするラムダ式を使用しています.

Original:

This example uses a ラムダ式 to count elements divisible by 3.

The text has been machine-translated via Google Translate.
You can help to correct and verify the translation. Click here for instructions.

#include <algorithm>#include <iostream>#include <vector>   int main(){int data[]={1, 2, 3, 4, 4, 3, 7, 8, 9, 10};std::vector<int> v(data, data+10);   int num_items1 = std::count_if(v.begin(), v.end(), [](int i){return i %3==0;});   std::cout<<"number divisible by three: "<< num_items1 <<'\n';}

出力:

number divisible by three: 3