std::count_if

template <class InputIterator, class UnaryPredicate>  typename iterator_traits<InputIterator>::difference_type    count_if (InputIterator first, InputIterator last, UnaryPredicate pred);

Return number of elements in range satisfying condition

Returns the number of elements in the range[first,last) for whichpred is true.

The behavior of this function template is equivalent to:

template <class InputIterator,class UnaryPredicate>typename iterator_traits<InputIterator>::difference_type    count_if (InputIterator first, InputIterator last, UnaryPredicate pred){typename iterator_traits<InputIterator>::difference_type ret = 0;while (first!=last) {if (pred(*first)) ++ret;    ++first;  }return ret;}

Parameters

first, last: Input iterators to the initial and final positions of the sequence of elements. The range used is[first,last), which contains all the elements betweenfirst andlast, including the element pointed byfirst but not the element pointed bylast.
pred: Unary function that accepts an element in the range as argument, and returns a value convertible tobool. The value returned indicates whether the element is counted by this function.
The function shall not modify its argument.
This can either be a function pointer or a function object.

Return value

The number of elements in the range[first,last) for whichpred does not returnfalse.
The return type (iterator_traits<InputIterator>::difference_type) is a signed integral type.

Example

// count_if example#include <iostream>// std::cout#include <algorithm>// std::count_if#include <vector>// std::vectorbool IsOdd (int i) {return ((i%2)==1); }int main () {  std::vector<int> myvector;for (int i=1; i<10; i++) myvector.push_back(i);// myvector: 1 2 3 4 5 6 7 8 9int mycount = count_if (myvector.begin(), myvector.end(), IsOdd);  std::cout <<"myvector contains " << mycount  <<" odd values.\n";return 0;}

Output:

myvector contains 5 odd values.

Complexity

Linear in thedistance betweenfirst andlast: Callspred once for each element.

Data races

The objects in the range[first,last) are accessed (each object is accessed exactly once).

Exceptions

Throws ifpred throws or if any of the operations on iterators throws.
Note that invalid arguments causeundefined behavior.

Movatterモバイル変換

C++

Reference

C library:

Containers:

Input/Output:

Multi-threading:

Other:

<algorithm>