=====count_if===== Syntax:


    #include 
    typename iterator_traits::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(), 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(), target_value, _1) );

''count_if'' can also be used on containers such as [[/stl/vector/|vectors]]:


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