stl:algorithm:count

count_if

Syntax:

    #include <algorithm>
    typename iterator_traits<input_iterator>::difference_type
    count_if( input_iterator start, input_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

Alternatively, the same result can be achieved with the boost::bind:

   int num_items = count_if( nums+start,
                             nums+end,
                             boost::bind(equal_to<int>(), target_value, _1) );

count_if can also be used on containers such as vectors:

   int num_items = count_if( nums.begin(),
                             nums.end(),
                             boost::bind(equal_to<int>(), target_value, _1) );