std::count, std::count_if

#include <algorithm>
#include <array>
#include <cassert>
#include <complex>
#include <iostream>
#include <iterator>
 
int main()
{
 constexpr std::array v{1, 2, 3, 4, 4, 3, 7, 8, 9, 10};
 std::cout << "v: ";
 std::copy (v.cbegin(), v.cend(), std::ostream_iterator <int>(std::cout, " "));
 std::cout << '\n';
 
 // Determine how many integers match a target value.
 for (const int target : {3, 4, 5})
 {
 const int num_items = std::count(v.cbegin(), v.cend(), target);
 std::cout << "number: " << target << ", count: " << num_items << '\n';
 }
 
 // Use a lambda expression to count elements divisible by 4.
 int count_div4 = std::count_if(v.begin(), v.end(), [](int i) { return i % 4 == 0; });
 std::cout << "numbers divisible by four: " << count_div4 << '\n';
 
 // A simplified version of `distance` with O(N) complexity:
 auto distance = [](auto first, auto last)
 {
 return std::count_if(first, last, [](auto) { return true; });
 };
 static_assert(distance(v.begin(), v.end()) == 10);
 
 std::array <std::complex <double>, 3> nums{{{4, 2}, {1, 3}, {4, 2}}};
 #ifdef __cpp_lib_algorithm_default_value_type
 // T gets deduced making list-initialization possible
 auto c = std::count(nums.cbegin(), nums.cend(), {4, 2});
 #else
 auto c = std::count(nums.cbegin(), nums.cend(), std::complex <double>{4, 2});
 #endif
 assert (c == 2);
}

v: 1 2 3 4 4 3 7 8 9 10
number: 3, count: 2
number: 4, count: 2
number: 5, count: 0
numbers divisible by four: 3