#include <algorithm>
  size_t count_if( iterator start, iterator end, UnaryPred p );

The count_if() function returns the number of elements between start and end for which the predicate p returns true.

For example, the following code uses count_if() with a predicate that returns true for the integer 3 to count the number of items in an array that are equal to 3:

 int nums[] = { 0, 1, 2, 3, 4, 5, 9, 3, 13 };
 int start = 0;
 int end = 9;           

 int target_value = 3;
 int num_items = count_if( nums+start,
                    nums+end,
                    bind2nd(equal_to<int>(), target_value) );             

 cout << "nums[] contains " << num_items << " items matching " << target_value << endl;               

When run, the above code displays the following output:

 nums[] contains 2 items matching 3