std::count

template <class InputIterator, class T>  typename iterator_traits<InputIterator>::difference_type    count (InputIterator first, InputIterator last, const T& val);

Count appearances of value in range

Returns the number of elements in the range [first,last) that compare equal to val.

The function uses operator== to compare the individual elements to val.

The behavior of this function template is equivalent to:

template <class InputIterator, class T>
  typename iterator_traits<InputIterator>::difference_type
    count (InputIterator first, InputIterator last, const T& val)
{
  typename iterator_traits<InputIterator>::difference_type ret = 0;
  while (first!=last) {
    if (*first == val) ++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.
val: Value to match.
T shall be a type supporting comparisons with the elements pointed by InputIterator using operator== (with the elements as left-hand side operands, and val as right-hand side).

Return value

The number of elements in the range [first,last) that compare equal to val.
The return type (iterator_traits<InputIterator>::difference_type) is a signed integral type.

Example

// count algorithm example
#include <iostream>     // std::cout
#include <algorithm>    // std::count
#include <vector>       // std::vector

int main () {
  // counting elements in array:
  int myints[] = {10,20,30,30,20,10,10,20};   // 8 elements
  int mycount = std::count (myints, myints+8, 10);
  std::cout << "10 appears " << mycount << " times.\n";

  // counting elements in container:
  std::vector<int> myvector (myints, myints+8);
  mycount = std::count (myvector.begin(), myvector.end(), 20);
  std::cout << "20 appears " << mycount  << " times.\n";

  return 0;
}

Output:

10 appears 3 times.
20 appears 3 times.

Complexity

Linear in the distance between first and last: Compares once each element.

Data races

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

Exceptions

Throws if either an element comparison or an operation on an iterator throws.
Note that invalid arguments cause undefined behavior.