cppreference.com > C++ Algorithms > count_if count_if Syntax:
      #include <algorithm>
      size_t count_if( iterator start, iterator end, BinPred 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
Related topics: count