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 which pred 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 between first and last, including the element pointed by first but not the element pointed by last.
pred: Unary function that accepts an element in the range as argument, and returns a value convertible to bool. 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 which pred does not return false.
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::vector

bool 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 9

  int 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 the distance between first and last: Calls pred once for each element.

Data races

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

Exceptions

Throws if pred throws or if any of the operations on iterators throws.
Note that invalid arguments cause undefined behavior.